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+Network Working Group                                 J. Klensin, Editor
+Request for Comments: 2821                             AT&T Laboratories
+Obsoletes: 821, 974, 1869                                     April 2001
+Updates: 1123
+Category: Standards Track
+
+
+                     Simple Mail Transfer Protocol
+
+Status of this Memo
+
+   This document specifies an Internet standards track protocol for the
+   Internet community, and requests discussion and suggestions for
+   improvements.  Please refer to the current edition of the "Internet
+   Official Protocol Standards" (STD 1) for the standardization state
+   and status of this protocol.  Distribution of this memo is unlimited.
+
+Copyright Notice
+
+   Copyright (C) The Internet Society (2001).  All Rights Reserved.
+
+Abstract
+
+   This document is a self-contained specification of the basic protocol
+   for the Internet electronic mail transport.  It consolidates, updates
+   and clarifies, but doesn't add new or change existing functionality
+   of the following:
+
+   -  the original SMTP (Simple Mail Transfer Protocol) specification of
+      RFC 821 [30],
+
+   -  domain name system requirements and implications for mail
+      transport from RFC 1035 [22] and RFC 974 [27],
+
+   -  the clarifications and applicability statements in RFC 1123 [2],
+      and
+
+   -  material drawn from the SMTP Extension mechanisms [19].
+
+   It obsoletes RFC 821, RFC 974, and updates RFC 1123 (replaces the
+   mail transport materials of RFC 1123).  However, RFC 821 specifies
+   some features that were not in significant use in the Internet by the
+   mid-1990s and (in appendices) some additional transport models.
+   Those sections are omitted here in the interest of clarity and
+   brevity; readers needing them should refer to RFC 821.
+
+
+
+
+
+
+Klensin                     Standards Track                     [Page 1]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   It also includes some additional material from RFC 1123 that required
+   amplification.  This material has been identified in multiple ways,
+   mostly by tracking flaming on various lists and newsgroups and
+   problems of unusual readings or interpretations that have appeared as
+   the SMTP extensions have been deployed.  Where this specification
+   moves beyond consolidation and actually differs from earlier
+   documents, it supersedes them technically as well as textually.
+
+   Although SMTP was designed as a mail transport and delivery protocol,
+   this specification also contains information that is important to its
+   use as a 'mail submission' protocol, as recommended for POP [3, 26]
+   and IMAP [6].  Additional submission issues are discussed in RFC 2476
+   [15].
+
+   Section 2.3 provides definitions of terms specific to this document.
+   Except when the historical terminology is necessary for clarity, this
+   document uses the current 'client' and 'server' terminology to
+   identify the sending and receiving SMTP processes, respectively.
+
+   A companion document [32] discusses message headers, message bodies
+   and formats and structures for them, and their relationship.
+
+Table of Contents
+
+   1. Introduction ..................................................  4
+   2. The SMTP Model ................................................  5
+   2.1 Basic Structure ..............................................  5
+   2.2 The Extension Model ..........................................  7
+   2.2.1 Background .................................................  7
+   2.2.2 Definition and Registration of Extensions ..................  8
+   2.3 Terminology ..................................................  9
+   2.3.1 Mail Objects ............................................... 10
+   2.3.2 Senders and Receivers ...................................... 10
+   2.3.3 Mail Agents and Message Stores ............................. 10
+   2.3.4 Host ....................................................... 11
+   2.3.5 Domain ..................................................... 11
+   2.3.6 Buffer and State Table ..................................... 11
+   2.3.7 Lines ...................................................... 12
+   2.3.8 Originator, Delivery, Relay, and Gateway Systems ........... 12
+   2.3.9 Message Content and Mail Data .............................. 13
+   2.3.10 Mailbox and Address ....................................... 13
+   2.3.11 Reply ..................................................... 13
+   2.4 General Syntax Principles and Transaction Model .............. 13
+   3. The SMTP Procedures: An Overview .............................. 15
+   3.1 Session Initiation ........................................... 15
+   3.2 Client Initiation ............................................ 16
+   3.3 Mail Transactions ............................................ 16
+   3.4 Forwarding for Address Correction or Updating ................ 19
+
+
+
+Klensin                     Standards Track                     [Page 2]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   3.5 Commands for Debugging Addresses ............................. 20
+   3.5.1 Overview ................................................... 20
+   3.5.2 VRFY Normal Response ....................................... 22
+   3.5.3 Meaning of VRFY or EXPN Success Response ................... 22
+   3.5.4 Semantics and Applications of EXPN ......................... 23
+   3.6 Domains ...................................................... 23
+   3.7 Relaying ..................................................... 24
+   3.8 Mail Gatewaying .............................................. 25
+   3.8.1 Header Fields in Gatewaying ................................ 26
+   3.8.2 Received Lines in Gatewaying ............................... 26
+   3.8.3 Addresses in Gatewaying .................................... 26
+   3.8.4 Other Header Fields in Gatewaying .......................... 27
+   3.8.5 Envelopes in Gatewaying .................................... 27
+   3.9 Terminating Sessions and Connections ......................... 27
+   3.10 Mailing Lists and Aliases ................................... 28
+   3.10.1 Alias ..................................................... 28
+   3.10.2 List ...................................................... 28
+   4. The SMTP Specifications ....................................... 29
+   4.1 SMTP Commands ................................................ 29
+   4.1.1 Command Semantics and Syntax ............................... 29
+   4.1.1.1  Extended HELLO (EHLO) or HELLO (HELO) ................... 29
+   4.1.1.2 MAIL (MAIL) .............................................. 31
+   4.1.1.3 RECIPIENT (RCPT) ......................................... 31
+   4.1.1.4 DATA (DATA) .............................................. 33
+   4.1.1.5 RESET (RSET) ............................................. 34
+   4.1.1.6 VERIFY (VRFY) ............................................ 35
+   4.1.1.7 EXPAND (EXPN) ............................................ 35
+   4.1.1.8 HELP (HELP) .............................................. 35
+   4.1.1.9 NOOP (NOOP) .............................................. 35
+   4.1.1.10 QUIT (QUIT) ............................................. 36
+   4.1.2 Command Argument Syntax .................................... 36
+   4.1.3 Address Literals ........................................... 38
+   4.1.4 Order of Commands .......................................... 39
+   4.1.5 Private-use Commands ....................................... 40
+   4.2  SMTP Replies ................................................ 40
+   4.2.1 Reply Code Severities and Theory ........................... 42
+   4.2.2 Reply Codes by Function Groups ............................. 44
+   4.2.3  Reply Codes in Numeric Order .............................. 45
+   4.2.4 Reply Code 502 ............................................. 46
+   4.2.5 Reply Codes After DATA and the Subsequent <CRLF>.<CRLF> .... 46
+   4.3 Sequencing of Commands and Replies ........................... 47
+   4.3.1 Sequencing Overview ........................................ 47
+   4.3.2 Command-Reply Sequences .................................... 48
+   4.4 Trace Information ............................................ 49
+   4.5 Additional Implementation Issues ............................. 53
+   4.5.1 Minimum Implementation ..................................... 53
+   4.5.2 Transparency ............................................... 53
+   4.5.3 Sizes and Timeouts ......................................... 54
+
+
+
+Klensin                     Standards Track                     [Page 3]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   4.5.3.1 Size limits and minimums ................................. 54
+   4.5.3.2 Timeouts ................................................. 56
+   4.5.4 Retry Strategies ........................................... 57
+   4.5.4.1 Sending Strategy ......................................... 58
+   4.5.4.2 Receiving Strategy ....................................... 59
+   4.5.5 Messages with a null reverse-path .......................... 59
+   5. Address Resolution and Mail Handling .......................... 60
+   6. Problem Detection and Handling ................................ 62
+   6.1 Reliable Delivery and Replies by Email ....................... 62
+   6.2 Loop Detection ............................................... 63
+   6.3 Compensating for Irregularities .............................. 63
+   7. Security Considerations ....................................... 64
+   7.1 Mail Security and Spoofing ................................... 64
+   7.2 "Blind" Copies ............................................... 65
+   7.3 VRFY, EXPN, and Security ..................................... 65
+   7.4 Information Disclosure in Announcements ...................... 66
+   7.5 Information Disclosure in Trace Fields ....................... 66
+   7.6 Information Disclosure in Message Forwarding ................. 67
+   7.7 Scope of Operation of SMTP Servers ........................... 67
+   8. IANA Considerations ........................................... 67
+   9. References .................................................... 68
+   10. Editor's Address ............................................. 70
+   11. Acknowledgments .............................................. 70
+   Appendices ....................................................... 71
+   A. TCP Transport Service ......................................... 71
+   B. Generating SMTP Commands from RFC 822 Headers ................. 71
+   C. Source Routes ................................................. 72
+   D. Scenarios ..................................................... 73
+   E. Other Gateway Issues .......................................... 76
+   F. Deprecated Features of RFC 821 ................................ 76
+   Full Copyright Statement ......................................... 79
+
+1. Introduction
+
+   The objective of the Simple Mail Transfer Protocol (SMTP) is to
+   transfer mail reliably and efficiently.
+
+   SMTP is independent of the particular transmission subsystem and
+   requires only a reliable ordered data stream channel.  While this
+   document specifically discusses transport over TCP, other transports
+   are possible.  Appendices to RFC 821 describe some of them.
+
+   An important feature of SMTP is its capability to transport mail
+   across networks, usually referred to as "SMTP mail relaying" (see
+   section 3.8).  A network consists of the mutually-TCP-accessible
+   hosts on the public Internet, the mutually-TCP-accessible hosts on a
+   firewall-isolated TCP/IP Intranet, or hosts in some other LAN or WAN
+   environment utilizing a non-TCP transport-level protocol.  Using
+
+
+
+Klensin                     Standards Track                     [Page 4]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   SMTP, a process can transfer mail to another process on the same
+   network or to some other network via a relay or gateway process
+   accessible to both networks.
+
+   In this way, a mail message may pass through a number of intermediate
+   relay or gateway hosts on its path from sender to ultimate recipient.
+   The Mail eXchanger mechanisms of the domain name system [22, 27] (and
+   section 5 of this document) are used to identify the appropriate
+   next-hop destination for a message being transported.
+
+2. The SMTP Model
+
+2.1 Basic Structure
+
+   The SMTP design can be pictured as:
+
+               +----------+                +----------+
+   +------+    |          |                |          |
+   | User |<-->|          |      SMTP      |          |
+   +------+    |  Client- |Commands/Replies| Server-  |
+   +------+    |   SMTP   |<-------------->|    SMTP  |    +------+
+   | File |<-->|          |    and Mail    |          |<-->| File |
+   |System|    |          |                |          |    |System|
+   +------+    +----------+                +----------+    +------+
+                SMTP client                SMTP server
+
+   When an SMTP client has a message to transmit, it establishes a two-
+   way transmission channel to an SMTP server.  The responsibility of an
+   SMTP client is to transfer mail messages to one or more SMTP servers,
+   or report its failure to do so.
+
+   The means by which a mail message is presented to an SMTP client, and
+   how that client determines the domain name(s) to which mail messages
+   are to be transferred is a local matter, and is not addressed by this
+   document.  In some cases, the domain name(s) transferred to, or
+   determined by, an SMTP client will identify the final destination(s)
+   of the mail message.  In other cases, common with SMTP clients
+   associated with implementations of the POP [3, 26] or IMAP [6]
+   protocols, or when the SMTP client is inside an isolated transport
+   service environment, the domain name determined will identify an
+   intermediate destination through which all mail messages are to be
+   relayed.  SMTP clients that transfer all traffic, regardless of the
+   target domain names associated with the individual messages, or that
+   do not maintain queues for retrying message transmissions that
+   initially cannot be completed, may otherwise conform to this
+   specification but are not considered fully-capable.  Fully-capable
+   SMTP implementations, including the relays used by these less capable
+
+
+
+
+Klensin                     Standards Track                     [Page 5]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   ones, and their destinations, are expected to support all of the
+   queuing, retrying, and alternate address functions discussed in this
+   specification.
+
+   The means by which an SMTP client, once it has determined a target
+   domain name, determines the identity of an SMTP server to which a
+   copy of a message is to be transferred, and then performs that
+   transfer, is covered by this document.  To effect a mail transfer to
+   an SMTP server, an SMTP client establishes a two-way transmission
+   channel to that SMTP server.  An SMTP client determines the address
+   of an appropriate host running an SMTP server by resolving a
+   destination domain name to either an intermediate Mail eXchanger host
+   or a final target host.
+
+   An SMTP server may be either the ultimate destination or an
+   intermediate "relay" (that is, it may assume the role of an SMTP
+   client after receiving the message) or "gateway" (that is, it may
+   transport the message further using some protocol other than SMTP).
+   SMTP commands are generated by the SMTP client and sent to the SMTP
+   server.  SMTP replies are sent from the SMTP server to the SMTP
+   client in response to the commands.
+
+   In other words, message transfer can occur in a single connection
+   between the original SMTP-sender and the final SMTP-recipient, or can
+   occur in a series of hops through intermediary systems.  In either
+   case, a formal handoff of responsibility for the message occurs: the
+   protocol requires that a server accept responsibility for either
+   delivering a message or properly reporting the failure to do so.
+
+   Once the transmission channel is established and initial handshaking
+   completed, the SMTP client normally initiates a mail transaction.
+   Such a transaction consists of a series of commands to specify the
+   originator and destination of the mail and transmission of the
+   message content (including any headers or other structure) itself.
+   When the same message is sent to multiple recipients, this protocol
+   encourages the transmission of only one copy of the data for all
+   recipients at the same destination (or intermediate relay) host.
+
+   The server responds to each command with a reply; replies may
+   indicate that the command was accepted, that additional commands are
+   expected, or that a temporary or permanent error condition exists.
+   Commands specifying the sender or recipients may include server-
+   permitted SMTP service extension requests as discussed in section
+   2.2.  The dialog is purposely lock-step, one-at-a-time, although this
+   can be modified by mutually-agreed extension requests such as command
+   pipelining [13].
+
+
+
+
+
+Klensin                     Standards Track                     [Page 6]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   Once a given mail message has been transmitted, the client may either
+   request that the connection be shut down or may initiate other mail
+   transactions.  In addition, an SMTP client may use a connection to an
+   SMTP server for ancillary services such as verification of email
+   addresses or retrieval of mailing list subscriber addresses.
+
+   As suggested above, this protocol provides mechanisms for the
+   transmission of mail.  This transmission normally occurs directly
+   from the sending user's host to the receiving user's host when the
+   two hosts are connected to the same transport service.  When they are
+   not connected to the same transport service, transmission occurs via
+   one or more relay SMTP servers.  An intermediate host that acts as
+   either an SMTP relay or as a gateway into some other transmission
+   environment is usually selected through the use of the domain name
+   service (DNS) Mail eXchanger mechanism.
+
+   Usually, intermediate hosts are determined via the DNS MX record, not
+   by explicit "source" routing (see section 5 and appendices C and
+   F.2).
+
+2.2 The Extension Model
+
+2.2.1 Background
+
+   In an effort that started in 1990, approximately a decade after RFC
+   821 was completed, the protocol was modified with a "service
+   extensions" model that permits the client and server to agree to
+   utilize shared functionality beyond the original SMTP requirements.
+   The SMTP extension mechanism defines a means whereby an extended SMTP
+   client and server may recognize each other, and the server can inform
+   the client as to the service extensions that it supports.
+
+   Contemporary SMTP implementations MUST support the basic extension
+   mechanisms.  For instance, servers MUST support the EHLO command even
+   if they do not implement any specific extensions and clients SHOULD
+   preferentially utilize EHLO rather than HELO.  (However, for
+   compatibility with older conforming implementations, SMTP clients and
+   servers MUST support the original HELO mechanisms as a fallback.)
+   Unless the different characteristics of HELO must be identified for
+   interoperability purposes, this document discusses only EHLO.
+
+   SMTP is widely deployed and high-quality implementations have proven
+   to be very robust.  However, the Internet community now considers
+   some services to be important that were not anticipated when the
+   protocol was first designed.  If support for those services is to be
+   added, it must be done in a way that permits older implementations to
+   continue working acceptably.  The extension framework consists of:
+
+
+
+
+Klensin                     Standards Track                     [Page 7]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   -  The SMTP command EHLO, superseding the earlier HELO,
+
+   -  a registry of SMTP service extensions,
+
+   -  additional parameters to the SMTP MAIL and RCPT commands, and
+
+   -  optional replacements for commands defined in this protocol, such
+      as for DATA in non-ASCII transmissions [33].
+
+   SMTP's strength comes primarily from its simplicity.  Experience with
+   many protocols has shown that protocols with few options tend towards
+   ubiquity, whereas protocols with many options tend towards obscurity.
+
+   Each and every extension, regardless of its benefits, must be
+   carefully scrutinized with respect to its implementation, deployment,
+   and interoperability costs.  In many cases, the cost of extending the
+   SMTP service will likely outweigh the benefit.
+
+2.2.2 Definition and Registration of Extensions
+
+   The IANA maintains a registry of SMTP service extensions.  A
+   corresponding EHLO keyword value is associated with each extension.
+   Each service extension registered with the IANA must be defined in a
+   formal standards-track or IESG-approved experimental protocol
+   document.  The definition must include:
+
+   -  the textual name of the SMTP service extension;
+
+   -  the EHLO keyword value associated with the extension;
+
+   -  the syntax and possible values of parameters associated with the
+      EHLO keyword value;
+
+   -  any additional SMTP verbs associated with the extension
+      (additional verbs will usually be, but are not required to be, the
+      same as the EHLO keyword value);
+
+   -  any new parameters the extension associates with the MAIL or RCPT
+      verbs;
+
+   -  a description of how support for the extension affects the
+      behavior of a server and client SMTP; and,
+
+   -  the increment by which the extension is increasing the maximum
+      length of the commands MAIL and/or RCPT, over that specified in
+      this standard.
+
+
+
+
+
+Klensin                     Standards Track                     [Page 8]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   In addition, any EHLO keyword value starting with an upper or lower
+   case "X" refers to a local SMTP service extension used exclusively
+   through bilateral agreement.  Keywords beginning with "X" MUST NOT be
+   used in a registered service extension.  Conversely, keyword values
+   presented in the EHLO response that do not begin with "X" MUST
+   correspond to a standard, standards-track, or IESG-approved
+   experimental SMTP service extension registered with IANA.  A
+   conforming server MUST NOT offer non-"X"-prefixed keyword values that
+   are not described in a registered extension.
+
+   Additional verbs and parameter names are bound by the same rules as
+   EHLO keywords; specifically, verbs beginning with "X" are local
+   extensions that may not be registered or standardized.  Conversely,
+   verbs not beginning with "X" must always be registered.
+
+2.3 Terminology
+
+   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
+   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
+   document are to be interpreted as described below.
+
+   1. MUST   This word, or the terms "REQUIRED" or "SHALL", mean that
+      the definition is an absolute requirement of the specification.
+
+   2. MUST NOT   This phrase, or the phrase "SHALL NOT", mean that the
+      definition is an absolute prohibition of the specification.
+
+   3. SHOULD   This word, or the adjective "RECOMMENDED", mean that
+      there may exist valid reasons in particular circumstances to
+      ignore a particular item, but the full implications must be
+      understood and carefully weighed before choosing a different
+      course.
+
+   4. SHOULD NOT   This phrase, or the phrase "NOT RECOMMENDED" mean
+      that there may exist valid reasons in particular circumstances
+      when the particular behavior is acceptable or even useful, but the
+      full implications should be understood and the case carefully
+      weighed before implementing any behavior described with this
+      label.
+
+   5. MAY   This word, or the adjective "OPTIONAL", mean that an item is
+      truly optional.  One vendor may choose to include the item because
+      a particular marketplace requires it or because the vendor feels
+      that it enhances the product while another vendor may omit the
+      same item.  An implementation which does not include a particular
+      option MUST be prepared to interoperate with another
+      implementation which does include the option, though perhaps with
+      reduced functionality.  In the same vein an implementation which
+
+
+
+Klensin                     Standards Track                     [Page 9]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+      does include a particular option MUST be prepared to interoperate
+      with another implementation which does not include the option
+      (except, of course, for the feature the option provides.)
+
+2.3.1 Mail Objects
+
+   SMTP transports a mail object.  A mail object contains an envelope
+   and content.
+
+   The SMTP envelope is sent as a series of SMTP protocol units
+   (described in section 3).  It consists of an originator address (to
+   which error reports should be directed); one or more recipient
+   addresses; and optional protocol extension material.  Historically,
+   variations on the recipient address specification command (RCPT TO)
+   could be used to specify alternate delivery modes, such as immediate
+   display; those variations have now been deprecated (see appendix F,
+   section F.6).
+
+   The SMTP content is sent in the SMTP DATA protocol unit and has two
+   parts:  the headers and the body.  If the content conforms to other
+   contemporary standards, the headers form a collection of field/value
+   pairs structured as in the message format specification [32]; the
+   body, if structured, is defined according to MIME [12].  The content
+   is textual in nature, expressed using the US-ASCII repertoire [1].
+   Although SMTP extensions (such as "8BITMIME" [20]) may relax this
+   restriction for the content body, the content headers are always
+   encoded using the US-ASCII repertoire.  A MIME extension [23] defines
+   an algorithm for representing header values outside the US-ASCII
+   repertoire, while still encoding them using the US-ASCII repertoire.
+
+2.3.2 Senders and Receivers
+
+   In RFC 821, the two hosts participating in an SMTP transaction were
+   described as the "SMTP-sender" and "SMTP-receiver".  This document
+   has been changed to reflect current industry terminology and hence
+   refers to them as the "SMTP client" (or sometimes just "the client")
+   and "SMTP server" (or just "the server"), respectively.  Since a
+   given host may act both as server and client in a relay situation,
+   "receiver" and "sender" terminology is still used where needed for
+   clarity.
+
+2.3.3 Mail Agents and Message Stores
+
+   Additional mail system terminology became common after RFC 821 was
+   published and, where convenient, is used in this specification.  In
+   particular, SMTP servers and clients provide a mail transport service
+   and therefore act as "Mail Transfer Agents" (MTAs).  "Mail User
+   Agents" (MUAs or UAs) are normally thought of as the sources and
+
+
+
+Klensin                     Standards Track                    [Page 10]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   targets of mail.  At the source, an MUA might collect mail to be
+   transmitted from a user and hand it off to an MTA; the final
+   ("delivery") MTA would be thought of as handing the mail off to an
+   MUA (or at least transferring responsibility to it, e.g., by
+   depositing the message in a "message store").  However, while these
+   terms are used with at least the appearance of great precision in
+   other environments, the implied boundaries between MUAs and MTAs
+   often do not accurately match common, and conforming, practices with
+   Internet mail.  Hence, the reader should be cautious about inferring
+   the strong relationships and responsibilities that might be implied
+   if these terms were used elsewhere.
+
+2.3.4 Host
+
+   For the purposes of this specification, a host is a computer system
+   attached to the Internet (or, in some cases, to a private TCP/IP
+   network) and supporting the SMTP protocol.  Hosts are known by names
+   (see "domain"); identifying them by numerical address is discouraged.
+
+2.3.5 Domain
+
+   A domain (or domain name) consists of one or more dot-separated
+   components.  These components ("labels" in DNS terminology [22]) are
+   restricted for SMTP purposes to consist of a sequence of letters,
+   digits, and hyphens drawn from the ASCII character set [1].  Domain
+   names are used as names of hosts and of other entities in the domain
+   name hierarchy.  For example, a domain may refer to an alias (label
+   of a CNAME RR) or the label of Mail eXchanger records to be used to
+   deliver mail instead of representing a host name.  See [22] and
+   section 5 of this specification.
+
+   The domain name, as described in this document and in [22], is the
+   entire, fully-qualified name (often referred to as an "FQDN").  A
+   domain name that is not in FQDN form is no more than a local alias.
+   Local aliases MUST NOT appear in any SMTP transaction.
+
+2.3.6 Buffer and State Table
+
+   SMTP sessions are stateful, with both parties carefully maintaining a
+   common view of the current state.  In this document we model this
+   state by a virtual "buffer" and a "state table" on the server which
+   may be used by the client to, for example, "clear the buffer" or
+   "reset the state table," causing the information in the buffer to be
+   discarded and the state to be returned to some previous state.
+
+
+
+
+
+
+
+Klensin                     Standards Track                    [Page 11]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+2.3.7 Lines
+
+   SMTP commands and, unless altered by a service extension, message
+   data, are transmitted in "lines".  Lines consist of zero or more data
+   characters terminated by the sequence ASCII character "CR" (hex value
+   0D) followed immediately by ASCII character "LF" (hex value 0A).
+   This termination sequence is denoted as <CRLF> in this document.
+   Conforming implementations MUST NOT recognize or generate any other
+   character or character sequence as a line terminator.  Limits MAY be
+   imposed on line lengths by servers (see section 4.5.3).
+
+   In addition, the appearance of "bare" "CR" or "LF" characters in text
+   (i.e., either without the other) has a long history of causing
+   problems in mail implementations and applications that use the mail
+   system as a tool.  SMTP client implementations MUST NOT transmit
+   these characters except when they are intended as line terminators
+   and then MUST, as indicated above, transmit them only as a <CRLF>
+   sequence.
+
+2.3.8 Originator, Delivery, Relay, and Gateway Systems
+
+   This specification makes a distinction among four types of SMTP
+   systems, based on the role those systems play in transmitting
+   electronic mail.  An "originating" system (sometimes called an SMTP
+   originator) introduces mail into the Internet or, more generally,
+   into a transport service environment.  A "delivery" SMTP system is
+   one that receives mail from a transport service environment and
+   passes it to a mail user agent or deposits it in a message store
+   which a mail user agent is expected to subsequently access.  A
+   "relay" SMTP system (usually referred to just as a "relay") receives
+   mail from an SMTP client and transmits it, without modification to
+   the message data other than adding trace information, to another SMTP
+   server for further relaying or for delivery.
+
+   A "gateway" SMTP system (usually referred to just as a "gateway")
+   receives mail from a client system in one transport environment and
+   transmits it to a server system in another transport environment.
+   Differences in protocols or message semantics between the transport
+   environments on either side of a gateway may require that the gateway
+   system perform transformations to the message that are not permitted
+   to SMTP relay systems.  For the purposes of this specification,
+   firewalls that rewrite addresses should be considered as gateways,
+   even if SMTP is used on both sides of them (see [11]).
+
+
+
+
+
+
+
+
+Klensin                     Standards Track                    [Page 12]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+2.3.9 Message Content and Mail Data
+
+   The terms "message content" and "mail data" are used interchangeably
+   in this document to describe the material transmitted after the DATA
+   command is accepted and before the end of data indication is
+   transmitted.  Message content includes message headers and the
+   possibly-structured message body.  The MIME specification [12]
+   provides the standard mechanisms for structured message bodies.
+
+2.3.10 Mailbox and Address
+
+   As used in this specification, an "address" is a character string
+   that identifies a user to whom mail will be sent or a location into
+   which mail will be deposited.  The term "mailbox" refers to that
+   depository.  The two terms are typically used interchangeably unless
+   the distinction between the location in which mail is placed (the
+   mailbox) and a reference to it (the address) is important.  An
+   address normally consists of user and domain specifications.  The
+   standard mailbox naming convention is defined to be "local-
+   part@domain": contemporary usage permits a much broader set of
+   applications than simple "user names".  Consequently, and due to a
+   long history of problems when intermediate hosts have attempted to
+   optimize transport by modifying them, the local-part MUST be
+   interpreted and assigned semantics only by the host specified in the
+   domain part of the address.
+
+2.3.11 Reply
+
+   An SMTP reply is an acknowledgment (positive or negative) sent from
+   receiver to sender via the transmission channel in response to a
+   command.  The general form of a reply is a numeric completion code
+   (indicating failure or success) usually followed by a text string.
+   The codes are for use by programs and the text is usually intended
+   for human users.  Recent work [34] has specified further structuring
+   of the reply strings, including the use of supplemental and more
+   specific completion codes.
+
+2.4 General Syntax Principles and Transaction Model
+
+   SMTP commands and replies have a rigid syntax.  All commands begin
+   with a command verb.  All Replies begin with a three digit numeric
+   code.  In some commands and replies, arguments MUST follow the verb
+   or reply code.  Some commands do not accept arguments (after the
+   verb), and some reply codes are followed, sometimes optionally, by
+   free form text.  In both cases, where text appears, it is separated
+   from the verb or reply code by a space character.  Complete
+   definitions of commands and replies appear in section 4.
+
+
+
+
+Klensin                     Standards Track                    [Page 13]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   Verbs and argument values (e.g., "TO:" or "to:" in the RCPT command
+   and extension name keywords) are not case sensitive, with the sole
+   exception in this specification of a mailbox local-part (SMTP
+   Extensions may explicitly specify case-sensitive elements).  That is,
+   a command verb, an argument value other than a mailbox local-part,
+   and free form text MAY be encoded in upper case, lower case, or any
+   mixture of upper and lower case with no impact on its meaning.  This
+   is NOT true of a mailbox local-part.  The local-part of a mailbox
+   MUST BE treated as case sensitive.  Therefore, SMTP implementations
+   MUST take care to preserve the case of mailbox local-parts.  Mailbox
+   domains are not case sensitive.  In particular, for some hosts the
+   user "smith" is different from the user "Smith".  However, exploiting
+   the case sensitivity of mailbox local-parts impedes interoperability
+   and is discouraged.
+
+   A few SMTP servers, in violation of this specification (and RFC 821)
+   require that command verbs be encoded by clients in upper case.
+   Implementations MAY wish to employ this encoding to accommodate those
+   servers.
+
+   The argument field consists of a variable length character string
+   ending with the end of the line, i.e., with the character sequence
+   <CRLF>.  The receiver will take no action until this sequence is
+   received.
+
+   The syntax for each command is shown with the discussion of that
+   command.  Common elements and parameters are shown in section 4.1.2.
+
+   Commands and replies are composed of characters from the ASCII
+   character set [1].  When the transport service provides an 8-bit byte
+   (octet) transmission channel, each 7-bit character is transmitted
+   right justified in an octet with the high order bit cleared to zero.
+   More specifically, the unextended SMTP service provides seven bit
+   transport only.  An originating SMTP client which has not
+   successfully negotiated an appropriate extension with a particular
+   server MUST NOT transmit messages with information in the high-order
+   bit of octets.  If such messages are transmitted in violation of this
+   rule, receiving SMTP servers MAY clear the high-order bit or reject
+   the message as invalid.  In general, a relay SMTP SHOULD assume that
+   the message content it has received is valid and, assuming that the
+   envelope permits doing so, relay it without inspecting that content.
+   Of course, if the content is mislabeled and the data path cannot
+   accept the actual content, this may result in ultimate delivery of a
+   severely garbled message to the recipient.  Delivery SMTP systems MAY
+   reject ("bounce") such messages rather than deliver them.  No sending
+   SMTP system is permitted to send envelope commands in any character
+
+
+
+
+
+Klensin                     Standards Track                    [Page 14]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   set other than US-ASCII; receiving systems SHOULD reject such
+   commands, normally using "500 syntax error - invalid character"
+   replies.
+
+   Eight-bit message content transmission MAY be requested of the server
+   by a client using extended SMTP facilities, notably the "8BITMIME"
+   extension [20].  8BITMIME SHOULD be supported by SMTP servers.
+   However, it MUST not be construed as authorization to transmit
+   unrestricted eight bit material.  8BITMIME MUST NOT be requested by
+   senders for material with the high bit on that is not in MIME format
+   with an appropriate content-transfer encoding; servers MAY reject
+   such messages.
+
+   The metalinguistic notation used in this document corresponds to the
+   "Augmented BNF" used in other Internet mail system documents.  The
+   reader who is not familiar with that syntax should consult the ABNF
+   specification [8].  Metalanguage terms used in running text are
+   surrounded by pointed brackets (e.g., <CRLF>) for clarity.
+
+3. The SMTP Procedures: An Overview
+
+   This section contains descriptions of the procedures used in SMTP:
+   session initiation, the mail transaction, forwarding mail, verifying
+   mailbox names and expanding mailing lists, and the opening and
+   closing exchanges.  Comments on relaying, a note on mail domains, and
+   a discussion of changing roles are included at the end of this
+   section.  Several complete scenarios are presented in appendix D.
+
+3.1 Session Initiation
+
+   An SMTP session is initiated when a client opens a connection to a
+   server and the server responds with an opening message.
+
+   SMTP server implementations MAY include identification of their
+   software and version information in the connection greeting reply
+   after the 220 code, a practice that permits more efficient isolation
+   and repair of any problems.  Implementations MAY make provision for
+   SMTP servers to disable the software and version announcement where
+   it causes security concerns.  While some systems also identify their
+   contact point for mail problems, this is not a substitute for
+   maintaining the required "postmaster" address (see section 4.5.1).
+
+   The SMTP protocol allows a server to formally reject a transaction
+   while still allowing the initial connection as follows: a 554
+   response MAY be given in the initial connection opening message
+   instead of the 220.  A server taking this approach MUST still wait
+   for the client to send a QUIT (see section 4.1.1.10) before closing
+   the connection and SHOULD respond to any intervening commands with
+
+
+
+Klensin                     Standards Track                    [Page 15]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   "503 bad sequence of commands".  Since an attempt to make an SMTP
+   connection to such a system is probably in error, a server returning
+   a 554 response on connection opening SHOULD provide enough
+   information in the reply text to facilitate debugging of the sending
+   system.
+
+3.2 Client Initiation
+
+   Once the server has sent the welcoming message and the client has
+   received it, the client normally sends the EHLO command to the
+   server, indicating the client's identity.  In addition to opening the
+   session, use of EHLO indicates that the client is able to process
+   service extensions and requests that the server provide a list of the
+   extensions it supports.  Older SMTP systems which are unable to
+   support service extensions and contemporary clients which do not
+   require service extensions in the mail session being initiated, MAY
+   use HELO instead of EHLO.  Servers MUST NOT return the extended
+   EHLO-style response to a HELO command.  For a particular connection
+   attempt, if the server returns a "command not recognized" response to
+   EHLO, the client SHOULD be able to fall back and send HELO.
+
+   In the EHLO command the host sending the command identifies itself;
+   the command may be interpreted as saying "Hello, I am <domain>" (and,
+   in the case of EHLO, "and I support service extension requests").
+
+3.3 Mail Transactions
+
+   There are three steps to SMTP mail transactions.  The transaction
+   starts with a MAIL command which gives the sender identification.
+   (In general, the MAIL command may be sent only when no mail
+   transaction is in progress; see section 4.1.4.)  A series of one or
+   more RCPT commands follows giving the receiver information.  Then a
+   DATA command initiates transfer of the mail data and is terminated by
+   the "end of mail" data indicator, which also confirms the
+   transaction.
+
+   The first step in the procedure is the MAIL command.
+
+      MAIL FROM:<reverse-path> [SP <mail-parameters> ] <CRLF>
+
+   This command tells the SMTP-receiver that a new mail transaction is
+   starting and to reset all its state tables and buffers, including any
+   recipients or mail data.  The <reverse-path> portion of the first or
+   only argument contains the source mailbox (between "<" and ">"
+   brackets), which can be used to report errors (see section 4.2 for a
+   discussion of error reporting).  If accepted, the SMTP server returns
+   a 250 OK reply.  If the mailbox specification is not acceptable for
+   some reason, the server MUST return a reply indicating whether the
+
+
+
+Klensin                     Standards Track                    [Page 16]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   failure is permanent (i.e., will occur again if the client tries to
+   send the same address again) or temporary (i.e., the address might be
+   accepted if the client tries again later).  Despite the apparent
+   scope of this requirement, there are circumstances in which the
+   acceptability of the reverse-path may not be determined until one or
+   more forward-paths (in RCPT commands) can be examined.  In those
+   cases, the server MAY reasonably accept the reverse-path (with a 250
+   reply) and then report problems after the forward-paths are received
+   and examined.  Normally, failures produce 550 or 553 replies.
+
+   Historically, the <reverse-path> can contain more than just a
+   mailbox, however, contemporary systems SHOULD NOT use source routing
+   (see appendix C).
+
+   The optional <mail-parameters> are associated with negotiated SMTP
+   service extensions (see section 2.2).
+
+   The second step in the procedure is the RCPT command.
+
+      RCPT TO:<forward-path> [ SP <rcpt-parameters> ] <CRLF>
+
+   The first or only argument to this command includes a forward-path
+   (normally a mailbox and domain, always surrounded by "<" and ">"
+   brackets) identifying one recipient.  If accepted, the SMTP server
+   returns a 250 OK reply and stores the forward-path.  If the recipient
+   is known not to be a deliverable address, the SMTP server returns a
+   550 reply, typically with a string such as "no such user - " and the
+   mailbox name (other circumstances and reply codes are possible).
+   This step of the procedure can be repeated any number of times.
+
+   The <forward-path> can contain more than just a mailbox.
+   Historically, the <forward-path> can be a source routing list of
+   hosts and the destination mailbox, however, contemporary SMTP clients
+   SHOULD NOT utilize source routes (see appendix C).  Servers MUST be
+   prepared to encounter a list of source routes in the forward path,
+   but SHOULD ignore the routes or MAY decline to support the relaying
+   they imply.  Similarly, servers MAY decline to accept mail that is
+   destined for other hosts or systems.  These restrictions make a
+   server useless as a relay for clients that do not support full SMTP
+   functionality.  Consequently, restricted-capability clients MUST NOT
+   assume that any SMTP server on the Internet can be used as their mail
+   processing (relaying) site.  If a RCPT command appears without a
+   previous MAIL command, the server MUST return a 503 "Bad sequence of
+   commands" response.  The optional <rcpt-parameters> are associated
+   with negotiated SMTP service extensions (see section 2.2).
+
+   The third step in the procedure is the DATA command (or some
+   alternative specified in a service extension).
+
+
+
+Klensin                     Standards Track                    [Page 17]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+      DATA <CRLF>
+
+   If accepted, the SMTP server returns a 354 Intermediate reply and
+   considers all succeeding lines up to but not including the end of
+   mail data indicator to be the message text.  When the end of text is
+   successfully received and stored the SMTP-receiver sends a 250 OK
+   reply.
+
+   Since the mail data is sent on the transmission channel, the end of
+   mail data must be indicated so that the command and reply dialog can
+   be resumed.  SMTP indicates the end of the mail data by sending a
+   line containing only a "." (period or full stop).  A transparency
+   procedure is used to prevent this from interfering with the user's
+   text (see section 4.5.2).
+
+   The end of mail data indicator also confirms the mail transaction and
+   tells the SMTP server to now process the stored recipients and mail
+   data.  If accepted, the SMTP server returns a 250 OK reply.  The DATA
+   command can fail at only two points in the protocol exchange:
+
+   -  If there was no MAIL, or no RCPT, command, or all such commands
+      were rejected, the server MAY return a "command out of sequence"
+      (503) or "no valid recipients" (554) reply in response to the DATA
+      command.  If one of those replies (or any other 5yz reply) is
+      received, the client MUST NOT send the message data; more
+      generally, message data MUST NOT be sent unless a 354 reply is
+      received.
+
+   -  If the verb is initially accepted and the 354 reply issued, the
+      DATA command should fail only if the mail transaction was
+      incomplete (for example, no recipients), or if resources were
+      unavailable (including, of course, the server unexpectedly
+      becoming unavailable), or if the server determines that the
+      message should be rejected for policy or other reasons.
+
+   However, in practice, some servers do not perform recipient
+   verification until after the message text is received.  These servers
+   SHOULD treat a failure for one or more recipients as a "subsequent
+   failure" and return a mail message as discussed in section 6.  Using
+   a "550 mailbox not found" (or equivalent) reply code after the data
+   are accepted makes it difficult or impossible for the client to
+   determine which recipients failed.
+
+   When RFC 822 format [7, 32] is being used, the mail data include the
+   memo header items such as Date, Subject, To, Cc, From.  Server SMTP
+   systems SHOULD NOT reject messages based on perceived defects in the
+   RFC 822 or MIME [12] message header or message body.  In particular,
+
+
+
+
+Klensin                     Standards Track                    [Page 18]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   they MUST NOT reject messages in which the numbers of Resent-fields
+   do not match or Resent-to appears without Resent-from and/or Resent-
+   date.
+
+   Mail transaction commands MUST be used in the order discussed above.
+
+3.4 Forwarding for Address Correction or Updating
+
+   Forwarding support is most often required to consolidate and simplify
+   addresses within, or relative to, some enterprise and less frequently
+   to establish addresses to link a person's prior address with current
+   one.  Silent forwarding of messages (without server notification to
+   the sender), for security or non-disclosure purposes, is common in
+   the contemporary Internet.
+
+   In both the enterprise and the "new address" cases, information
+   hiding (and sometimes security) considerations argue against exposure
+   of the "final" address through the SMTP protocol as a side-effect of
+   the forwarding activity.  This may be especially important when the
+   final address may not even be reachable by the sender.  Consequently,
+   the "forwarding" mechanisms described in section 3.2 of RFC 821, and
+   especially the 251 (corrected destination) and 551 reply codes from
+   RCPT must be evaluated carefully by implementers and, when they are
+   available, by those configuring systems.
+
+   In particular:
+
+   *  Servers MAY forward messages when they are aware of an address
+      change.  When they do so, they MAY either provide address-updating
+      information with a 251 code, or may forward "silently" and return
+      a 250 code.  But, if a 251 code is used, they MUST NOT assume that
+      the client will actually update address information or even return
+      that information to the user.
+
+   Alternately,
+
+   *  Servers MAY reject or bounce messages when they are not
+      deliverable when addressed.  When they do so, they MAY either
+      provide address-updating information with a 551 code, or may
+      reject the message as undeliverable with a 550 code and no
+      address-specific information.  But, if a 551 code is used, they
+      MUST NOT assume that the client will actually update address
+      information or even return that information to the user.
+
+   SMTP server implementations that support the 251 and/or 551 reply
+   codes are strongly encouraged to provide configuration mechanisms so
+   that sites which conclude that they would undesirably disclose
+   information can disable or restrict their use.
+
+
+
+Klensin                     Standards Track                    [Page 19]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+3.5 Commands for Debugging Addresses
+
+3.5.1 Overview
+
+   SMTP provides commands to verify a user name or obtain the content of
+   a mailing list.  This is done with the VRFY and EXPN commands, which
+   have character string arguments.  Implementations SHOULD support VRFY
+   and EXPN (however, see section 3.5.2 and 7.3).
+
+   For the VRFY command, the string is a user name or a user name and
+   domain (see below).  If a normal (i.e., 250) response is returned,
+   the response MAY include the full name of the user and MUST include
+   the mailbox of the user.  It MUST be in either of the following
+   forms:
+
+      User Name <local-part@domain>
+      local-part@domain
+
+   When a name that is the argument to VRFY could identify more than one
+   mailbox, the server MAY either note the ambiguity or identify the
+   alternatives.  In other words, any of the following are legitimate
+   response to VRFY:
+
+      553 User ambiguous
+
+   or
+
+      553- Ambiguous;  Possibilities are
+      553-Joe Smith <jsmith@foo.com>
+      553-Harry Smith <hsmith@foo.com>
+      553 Melvin Smith <dweep@foo.com>
+
+   or
+
+      553-Ambiguous;  Possibilities
+      553- <jsmith@foo.com>
+      553- <hsmith@foo.com>
+      553 <dweep@foo.com>
+
+   Under normal circumstances, a client receiving a 553 reply would be
+   expected to expose the result to the user.  Use of exactly the forms
+   given, and the "user ambiguous" or "ambiguous" keywords, possibly
+   supplemented by extended reply codes such as those described in [34],
+   will facilitate automated translation into other languages as needed.
+   Of course, a client that was highly automated or that was operating
+   in another language than English, might choose to try to translate
+   the response, to return some other indication to the user than the
+
+
+
+
+Klensin                     Standards Track                    [Page 20]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   literal text of the reply, or to take some automated action such as
+   consulting a directory service for additional information before
+   reporting to the user.
+
+   For the EXPN command, the string identifies a mailing list, and the
+   successful (i.e., 250) multiline response MAY include the full name
+   of the users and MUST give the mailboxes on the mailing list.
+
+   In some hosts the distinction between a mailing list and an alias for
+   a single mailbox is a bit fuzzy, since a common data structure may
+   hold both types of entries, and it is possible to have mailing lists
+   containing only one mailbox.  If a request is made to apply VRFY to a
+   mailing list, a positive response MAY be given if a message so
+   addressed would be delivered to everyone on the list, otherwise an
+   error SHOULD be reported (e.g., "550 That is a mailing list, not a
+   user" or "252 Unable to verify members of mailing list").  If a
+   request is made to expand a user name, the server MAY return a
+   positive response consisting of a list containing one name, or an
+   error MAY be reported (e.g., "550 That is a user name, not a mailing
+   list").
+
+   In the case of a successful multiline reply (normal for EXPN) exactly
+   one mailbox is to be specified on each line of the reply.  The case
+   of an ambiguous request is discussed above.
+
+   "User name" is a fuzzy term and has been used deliberately.  An
+   implementation of the VRFY or EXPN commands MUST include at least
+   recognition of local mailboxes as "user names".  However, since
+   current Internet practice often results in a single host handling
+   mail for multiple domains, hosts, especially hosts that provide this
+   functionality, SHOULD accept the "local-part@domain" form as a "user
+   name"; hosts MAY also choose to recognize other strings as "user
+   names".
+
+   The case of expanding a mailbox list requires a multiline reply, such
+   as:
+
+      C: EXPN Example-People
+      S: 250-Jon Postel <Postel@isi.edu>
+      S: 250-Fred Fonebone <Fonebone@physics.foo-u.edu>
+      S: 250 Sam Q. Smith <SQSmith@specific.generic.com>
+
+   or
+
+      C: EXPN Executive-Washroom-List
+      S: 550 Access Denied to You.
+
+
+
+
+
+Klensin                     Standards Track                    [Page 21]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   The character string arguments of the VRFY and EXPN commands cannot
+   be further restricted due to the variety of implementations of the
+   user name and mailbox list concepts.  On some systems it may be
+   appropriate for the argument of the EXPN command to be a file name
+   for a file containing a mailing list, but again there are a variety
+   of file naming conventions in the Internet.  Similarly, historical
+   variations in what is returned by these commands are such that the
+   response SHOULD be interpreted very carefully, if at all, and SHOULD
+   generally only be used for diagnostic purposes.
+
+3.5.2 VRFY Normal Response
+
+   When normal (2yz or 551) responses are returned from a VRFY or EXPN
+   request, the reply normally includes the mailbox name, i.e.,
+   "<local-part@domain>", where "domain" is a fully qualified domain
+   name, MUST appear in the syntax.  In circumstances exceptional enough
+   to justify violating the intent of this specification, free-form text
+   MAY be returned.  In order to facilitate parsing by both computers
+   and people, addresses SHOULD appear in pointed brackets.  When
+   addresses, rather than free-form debugging information, are returned,
+   EXPN and VRFY MUST return only valid domain addresses that are usable
+   in SMTP RCPT commands.  Consequently, if an address implies delivery
+   to a program or other system, the mailbox name used to reach that
+   target MUST be given.  Paths (explicit source routes) MUST NOT be
+   returned by VRFY or EXPN.
+
+   Server implementations SHOULD support both VRFY and EXPN.  For
+   security reasons, implementations MAY provide local installations a
+   way to disable either or both of these commands through configuration
+   options or the equivalent.  When these commands are supported, they
+   are not required to work across relays when relaying is supported.
+   Since they were both optional in RFC 821, they MUST be listed as
+   service extensions in an EHLO response, if they are supported.
+
+3.5.3 Meaning of VRFY or EXPN Success Response
+
+   A server MUST NOT return a 250 code in response to a VRFY or EXPN
+   command unless it has actually verified the address.  In particular,
+   a server MUST NOT return 250 if all it has done is to verify that the
+   syntax given is valid.  In that case, 502 (Command not implemented)
+   or 500 (Syntax error, command unrecognized) SHOULD be returned.  As
+   stated elsewhere, implementation (in the sense of actually validating
+   addresses and returning information) of VRFY and EXPN are strongly
+   recommended.  Hence, implementations that return 500 or 502 for VRFY
+   are not in full compliance with this specification.
+
+
+
+
+
+
+Klensin                     Standards Track                    [Page 22]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   There may be circumstances where an address appears to be valid but
+   cannot reasonably be verified in real time, particularly when a
+   server is acting as a mail exchanger for another server or domain.
+   "Apparent validity" in this case would normally involve at least
+   syntax checking and might involve verification that any domains
+   specified were ones to which the host expected to be able to relay
+   mail.  In these situations, reply code 252 SHOULD be returned.  These
+   cases parallel the discussion of RCPT verification discussed in
+   section 2.1.  Similarly, the discussion in section 3.4 applies to the
+   use of reply codes 251 and 551 with VRFY (and EXPN) to indicate
+   addresses that are recognized but that would be forwarded or bounced
+   were mail received for them.  Implementations generally SHOULD be
+   more aggressive about address verification in the case of VRFY than
+   in the case of RCPT, even if it takes a little longer to do so.
+
+3.5.4 Semantics and Applications of EXPN
+
+   EXPN is often very useful in debugging and understanding problems
+   with mailing lists and multiple-target-address aliases.  Some systems
+   have attempted to use source expansion of mailing lists as a means of
+   eliminating duplicates.  The propagation of aliasing systems with
+   mail on the Internet, for hosts (typically with MX and CNAME DNS
+   records), for mailboxes (various types of local host aliases), and in
+   various proxying arrangements, has made it nearly impossible for
+   these strategies to work consistently, and mail systems SHOULD NOT
+   attempt them.
+
+3.6 Domains
+
+   Only resolvable, fully-qualified, domain names (FQDNs) are permitted
+   when domain names are used in SMTP.  In other words, names that can
+   be resolved to MX RRs or A RRs (as discussed in section 5) are
+   permitted, as are CNAME RRs whose targets can be resolved, in turn,
+   to MX or A RRs.  Local nicknames or unqualified names MUST NOT be
+   used.  There are two exceptions to the rule requiring FQDNs:
+
+   -  The domain name given in the EHLO command MUST BE either a primary
+      host name (a domain name that resolves to an A RR) or, if the host
+      has no name, an address literal as described in section 4.1.1.1.
+
+   -  The reserved mailbox name "postmaster" may be used in a RCPT
+      command without domain qualification (see section 4.1.1.3) and
+      MUST be accepted if so used.
+
+
+
+
+
+
+
+
+Klensin                     Standards Track                    [Page 23]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+3.7 Relaying
+
+   In general, the availability of Mail eXchanger records in the domain
+   name system [22, 27] makes the use of explicit source routes in the
+   Internet mail system unnecessary.  Many historical problems with
+   their interpretation have made their use undesirable.  SMTP clients
+   SHOULD NOT generate explicit source routes except under unusual
+   circumstances.  SMTP servers MAY decline to act as mail relays or to
+   accept addresses that specify source routes.  When route information
+   is encountered, SMTP servers are also permitted to ignore the route
+   information and simply send to the final destination specified as the
+   last element in the route and SHOULD do so.  There has been an
+   invalid practice of using names that do not appear in the DNS as
+   destination names, with the senders counting on the intermediate
+   hosts specified in source routing to resolve any problems.  If source
+   routes are stripped, this practice will cause failures.  This is one
+   of several reasons why SMTP clients MUST NOT generate invalid source
+   routes or depend on serial resolution of names.
+
+   When source routes are not used, the process described in RFC 821 for
+   constructing a reverse-path from the forward-path is not applicable
+   and the reverse-path at the time of delivery will simply be the
+   address that appeared in the MAIL command.
+
+   A relay SMTP server is usually the target of a DNS MX record that
+   designates it, rather than the final delivery system.  The relay
+   server may accept or reject the task of relaying the mail in the same
+   way it accepts or rejects mail for a local user.  If it accepts the
+   task, it then becomes an SMTP client, establishes a transmission
+   channel to the next SMTP server specified in the DNS (according to
+   the rules in section 5), and sends it the mail.  If it declines to
+   relay mail to a particular address for policy reasons, a 550 response
+   SHOULD be returned.
+
+   Many mail-sending clients exist, especially in conjunction with
+   facilities that receive mail via POP3 or IMAP, that have limited
+   capability to support some of the requirements of this specification,
+   such as the ability to queue messages for subsequent delivery
+   attempts.  For these clients, it is common practice to make private
+   arrangements to send all messages to a single server for processing
+   and subsequent distribution.  SMTP, as specified here, is not ideally
+   suited for this role, and work is underway on standardized mail
+   submission protocols that might eventually supercede the current
+   practices.  In any event, because these arrangements are private and
+   fall outside the scope of this specification, they are not described
+   here.
+
+
+
+
+
+Klensin                     Standards Track                    [Page 24]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   It is important to note that MX records can point to SMTP servers
+   which act as gateways into other environments, not just SMTP relays
+   and final delivery systems; see sections 3.8 and 5.
+
+   If an SMTP server has accepted the task of relaying the mail and
+   later finds that the destination is incorrect or that the mail cannot
+   be delivered for some other reason, then it MUST construct an
+   "undeliverable mail" notification message and send it to the
+   originator of the undeliverable mail (as indicated by the reverse-
+   path).  Formats specified for non-delivery reports by other standards
+   (see, for example, [24, 25]) SHOULD be used if possible.
+
+   This notification message must be from the SMTP server at the relay
+   host or the host that first determines that delivery cannot be
+   accomplished.  Of course, SMTP servers MUST NOT send notification
+   messages about problems transporting notification messages.  One way
+   to prevent loops in error reporting is to specify a null reverse-path
+   in the MAIL command of a notification message.  When such a message
+   is transmitted the reverse-path MUST be set to null (see section
+   4.5.5 for additional discussion).  A MAIL command with a null
+   reverse-path appears as follows:
+
+      MAIL FROM:<>
+
+   As discussed in section 2.4.1, a relay SMTP has no need to inspect or
+   act upon the headers or body of the message data and MUST NOT do so
+   except to add its own "Received:" header (section 4.4) and,
+   optionally, to attempt to detect looping in the mail system (see
+   section 6.2).
+
+3.8 Mail Gatewaying
+
+   While the relay function discussed above operates within the Internet
+   SMTP transport service environment, MX records or various forms of
+   explicit routing may require that an intermediate SMTP server perform
+   a translation function between one transport service and another.  As
+   discussed in section 2.3.8, when such a system is at the boundary
+   between two transport service environments, we refer to it as a
+   "gateway" or "gateway SMTP".
+
+   Gatewaying mail between different mail environments, such as
+   different mail formats and protocols, is complex and does not easily
+   yield to standardization.  However, some general requirements may be
+   given for a gateway between the Internet and another mail
+   environment.
+
+
+
+
+
+
+Klensin                     Standards Track                    [Page 25]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+3.8.1 Header Fields in Gatewaying
+
+   Header fields MAY be rewritten when necessary as messages are
+   gatewayed across mail environment boundaries.  This may involve
+   inspecting the message body or interpreting the local-part of the
+   destination address in spite of the prohibitions in section 2.4.1.
+
+   Other mail systems gatewayed to the Internet often use a subset of
+   RFC 822 headers or provide similar functionality with a different
+   syntax, but some of these mail systems do not have an equivalent to
+   the SMTP envelope.  Therefore, when a message leaves the Internet
+   environment, it may be necessary to fold the SMTP envelope
+   information into the message header.  A possible solution would be to
+   create new header fields to carry the envelope information (e.g.,
+   "X-SMTP-MAIL:"  and "X-SMTP-RCPT:"); however, this would require
+   changes in mail programs in foreign environments and might risk
+   disclosure of private information (see section 7.2).
+
+3.8.2 Received Lines in Gatewaying
+
+   When forwarding a message into or out of the Internet environment, a
+   gateway MUST prepend a Received: line, but it MUST NOT alter in any
+   way a Received: line that is already in the header.
+
+   "Received:" fields of messages originating from other environments
+   may not conform exactly to this specification.  However, the most
+   important use of Received: lines is for debugging mail faults, and
+   this debugging can be severely hampered by well-meaning gateways that
+   try to "fix" a Received: line.  As another consequence of trace
+   fields arising in non-SMTP environments, receiving systems MUST NOT
+   reject mail based on the format of a trace field and SHOULD be
+   extremely robust in the light of unexpected information or formats in
+   those fields.
+
+   The gateway SHOULD indicate the environment and protocol in the "via"
+   clauses of Received field(s) that it supplies.
+
+3.8.3 Addresses in Gatewaying
+
+   From the Internet side, the gateway SHOULD accept all valid address
+   formats in SMTP commands and in RFC 822 headers, and all valid RFC
+   822 messages.  Addresses and headers generated by gateways MUST
+   conform to applicable Internet standards (including this one and RFC
+   822).  Gateways are, of course, subject to the same rules for
+   handling source routes as those described for other SMTP systems in
+   section 3.3.
+
+
+
+
+
+Klensin                     Standards Track                    [Page 26]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+3.8.4 Other Header Fields in Gatewaying
+
+   The gateway MUST ensure that all header fields of a message that it
+   forwards into the Internet mail environment meet the requirements for
+   Internet mail.  In particular, all addresses in "From:", "To:",
+   "Cc:", etc., fields MUST be transformed (if necessary) to satisfy RFC
+   822 syntax, MUST reference only fully-qualified domain names, and
+   MUST be effective and useful for sending replies.  The translation
+   algorithm used to convert mail from the Internet protocols to another
+   environment's protocol SHOULD ensure that error messages from the
+   foreign mail environment are delivered to the return path from the
+   SMTP envelope, not to the sender listed in the "From:" field (or
+   other fields) of the RFC 822 message.
+
+3.8.5 Envelopes in Gatewaying
+
+   Similarly, when forwarding a message from another environment into
+   the Internet, the gateway SHOULD set the envelope return path in
+   accordance with an error message return address, if supplied by the
+   foreign environment.  If the foreign environment has no equivalent
+   concept, the gateway must select and use a best approximation, with
+   the message originator's address as the default of last resort.
+
+3.9 Terminating Sessions and Connections
+
+   An SMTP connection is terminated when the client sends a QUIT
+   command.  The server responds with a positive reply code, after which
+   it closes the connection.
+
+   An SMTP server MUST NOT intentionally close the connection except:
+
+   -  After receiving a QUIT command and responding with a 221 reply.
+
+   -  After detecting the need to shut down the SMTP service and
+      returning a 421 response code.  This response code can be issued
+      after the server receives any command or, if necessary,
+      asynchronously from command receipt (on the assumption that the
+      client will receive it after the next command is issued).
+
+   In particular, a server that closes connections in response to
+   commands that are not understood is in violation of this
+   specification.  Servers are expected to be tolerant of unknown
+   commands, issuing a 500 reply and awaiting further instructions from
+   the client.
+
+
+
+
+
+
+
+Klensin                     Standards Track                    [Page 27]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   An SMTP server which is forcibly shut down via external means SHOULD
+   attempt to send a line containing a 421 response code to the SMTP
+   client before exiting.  The SMTP client will normally read the 421
+   response code after sending its next command.
+
+   SMTP clients that experience a connection close, reset, or other
+   communications failure due to circumstances not under their control
+   (in violation of the intent of this specification but sometimes
+   unavoidable) SHOULD, to maintain the robustness of the mail system,
+   treat the mail transaction as if a 451 response had been received and
+   act accordingly.
+
+3.10 Mailing Lists and Aliases
+
+   An SMTP-capable host SHOULD support both the alias and the list
+   models of address expansion for multiple delivery.  When a message is
+   delivered or forwarded to each address of an expanded list form, the
+   return address in the envelope ("MAIL FROM:") MUST be changed to be
+   the address of a person or other entity who administers the list.
+   However, in this case, the message header [32] MUST be left
+   unchanged; in particular, the "From" field of the message header is
+   unaffected.
+
+   An important mail facility is a mechanism for multi-destination
+   delivery of a single message, by transforming (or "expanding" or
+   "exploding") a pseudo-mailbox address into a list of destination
+   mailbox addresses.  When a message is sent to such a pseudo-mailbox
+   (sometimes called an "exploder"), copies are forwarded or
+   redistributed to each mailbox in the expanded list.  Servers SHOULD
+   simply utilize the addresses on the list; application of heuristics
+   or other matching rules to eliminate some addresses, such as that of
+   the originator, is strongly discouraged.  We classify such a pseudo-
+   mailbox as an "alias" or a "list", depending upon the expansion
+   rules.
+
+3.10.1 Alias
+
+   To expand an alias, the recipient mailer simply replaces the pseudo-
+   mailbox address in the envelope with each of the expanded addresses
+   in turn; the rest of the envelope and the message body are left
+   unchanged.  The message is then delivered or forwarded to each
+   expanded address.
+
+3.10.2 List
+
+   A mailing list may be said to operate by "redistribution" rather than
+   by "forwarding".  To expand a list, the recipient mailer replaces the
+   pseudo-mailbox address in the envelope with all of the expanded
+
+
+
+Klensin                     Standards Track                    [Page 28]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   addresses.  The return address in the envelope is changed so that all
+   error messages generated by the final deliveries will be returned to
+   a list administrator, not to the message originator, who generally
+   has no control over the contents of the list and will typically find
+   error messages annoying.
+
+4. The SMTP Specifications
+
+4.1 SMTP Commands
+
+4.1.1 Command Semantics and Syntax
+
+   The SMTP commands define the mail transfer or the mail system
+   function requested by the user.  SMTP commands are character strings
+   terminated by <CRLF>.  The commands themselves are alphabetic
+   characters terminated by <SP> if parameters follow and <CRLF>
+   otherwise.  (In the interest of improved interoperability, SMTP
+   receivers are encouraged to tolerate trailing white space before the
+   terminating <CRLF>.)  The syntax of the local part of a mailbox must
+   conform to receiver site conventions and the syntax specified in
+   section 4.1.2.  The SMTP commands are discussed below.  The SMTP
+   replies are discussed in section 4.2.
+
+   A mail transaction involves several data objects which are
+   communicated as arguments to different commands.  The reverse-path is
+   the argument of the MAIL command, the forward-path is the argument of
+   the RCPT command, and the mail data is the argument of the DATA
+   command.  These arguments or data objects must be transmitted and
+   held pending the confirmation communicated by the end of mail data
+   indication which finalizes the transaction.  The model for this is
+   that distinct buffers are provided to hold the types of data objects,
+   that is, there is a reverse-path buffer, a forward-path buffer, and a
+   mail data buffer.  Specific commands cause information to be appended
+   to a specific buffer, or cause one or more buffers to be cleared.
+
+   Several commands (RSET, DATA, QUIT) are specified as not permitting
+   parameters.  In the absence of specific extensions offered by the
+   server and accepted by the client, clients MUST NOT send such
+   parameters and servers SHOULD reject commands containing them as
+   having invalid syntax.
+
+4.1.1.1  Extended HELLO (EHLO) or HELLO (HELO)
+
+   These commands are used to identify the SMTP client to the SMTP
+   server.  The argument field contains the fully-qualified domain name
+   of the SMTP client if one is available.  In situations in which the
+   SMTP client system does not have a meaningful domain name (e.g., when
+   its address is dynamically allocated and no reverse mapping record is
+
+
+
+Klensin                     Standards Track                    [Page 29]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   available), the client SHOULD send an address literal (see section
+   4.1.3), optionally followed by information that will help to identify
+   the client system.  y The SMTP server identifies itself to the SMTP
+   client in the connection greeting reply and in the response to this
+   command.
+
+   A client SMTP SHOULD start an SMTP session by issuing the EHLO
+   command.  If the SMTP server supports the SMTP service extensions it
+   will give a successful response, a failure response, or an error
+   response.  If the SMTP server, in violation of this specification,
+   does not support any SMTP service extensions it will generate an
+   error response.  Older client SMTP systems MAY, as discussed above,
+   use HELO (as specified in RFC 821) instead of EHLO, and servers MUST
+   support the HELO command and reply properly to it.  In any event, a
+   client MUST issue HELO or EHLO before starting a mail transaction.
+
+   These commands, and a "250 OK" reply to one of them, confirm that
+   both the SMTP client and the SMTP server are in the initial state,
+   that is, there is no transaction in progress and all state tables and
+   buffers are cleared.
+
+   Syntax:
+
+      ehlo            = "EHLO" SP Domain CRLF
+      helo            = "HELO" SP Domain CRLF
+
+   Normally, the response to EHLO will be a multiline reply.  Each line
+   of the response contains a keyword and, optionally, one or more
+   parameters.  Following the normal syntax for multiline replies, these
+   keyworks follow the code (250) and a hyphen for all but the last
+   line, and the code and a space for the last line.  The syntax for a
+   positive response, using the ABNF notation and terminal symbols of
+   [8], is:
+
+      ehlo-ok-rsp  =    ( "250"    domain [ SP ehlo-greet ] CRLF )
+                   / (    "250-"   domain [ SP ehlo-greet ] CRLF
+                       *( "250-"   ehlo-line                CRLF )
+                          "250"    SP ehlo-line             CRLF  )
+
+      ehlo-greet   = 1*(%d0-9 / %d11-12 / %d14-127)
+                   ; string of any characters other than CR or LF
+
+      ehlo-line    = ehlo-keyword *( SP ehlo-param )
+
+      ehlo-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")
+                   ; additional syntax of ehlo-params depends on
+                   ; ehlo-keyword
+
+
+
+
+Klensin                     Standards Track                    [Page 30]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+      ehlo-param   = 1*(%d33-127)
+                   ; any CHAR excluding <SP> and all
+                   ; control characters (US-ASCII 0-31 inclusive)
+
+   Although EHLO keywords may be specified in upper, lower, or mixed
+   case, they MUST always be recognized and processed in a case-
+   insensitive manner.  This is simply an extension of practices
+   specified in RFC 821 and section 2.4.1.
+
+4.1.1.2 MAIL (MAIL)
+
+   This command is used to initiate a mail transaction in which the mail
+   data is delivered to an SMTP server which may, in turn, deliver it to
+   one or more mailboxes or pass it on to another system (possibly using
+   SMTP).  The argument field contains a reverse-path and may contain
+   optional parameters.  In general, the MAIL command may be sent only
+   when no mail transaction is in progress, see section 4.1.4.
+
+   The reverse-path consists of the sender mailbox.  Historically, that
+   mailbox might optionally have been preceded by a list of hosts, but
+   that behavior is now deprecated (see appendix C).  In some types of
+   reporting messages for which a reply is likely to cause a mail loop
+   (for example, mail delivery and nondelivery notifications), the
+   reverse-path may be null (see section 3.7).
+
+   This command clears the reverse-path buffer, the forward-path buffer,
+   and the mail data buffer; and inserts the reverse-path information
+   from this command into the reverse-path buffer.
+
+   If service extensions were negotiated, the MAIL command may also
+   carry parameters associated with a particular service extension.
+
+   Syntax:
+
+      "MAIL FROM:" ("<>" / Reverse-Path)
+                       [SP Mail-parameters] CRLF
+
+4.1.1.3 RECIPIENT (RCPT)
+
+   This command is used to identify an individual recipient of the mail
+   data; multiple recipients are specified by multiple use of this
+   command.  The argument field contains a forward-path and may contain
+   optional parameters.
+
+   The forward-path normally consists of the required destination
+   mailbox.  Sending systems SHOULD not generate the optional list of
+   hosts known as a source route.  Receiving systems MUST recognize
+
+
+
+
+Klensin                     Standards Track                    [Page 31]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   source route syntax but SHOULD strip off the source route
+   specification and utilize the domain name associated with the mailbox
+   as if the source route had not been provided.
+
+   Similarly, relay hosts SHOULD strip or ignore source routes, and
+   names MUST NOT be copied into the reverse-path.  When mail reaches
+   its ultimate destination (the forward-path contains only a
+   destination mailbox), the SMTP server inserts it into the destination
+   mailbox in accordance with its host mail conventions.
+
+   For example, mail received at relay host xyz.com with envelope
+   commands
+
+      MAIL FROM:<userx@y.foo.org>
+      RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org>
+
+   will normally be sent directly on to host d.bar.org with envelope
+   commands
+
+      MAIL FROM:<userx@y.foo.org>
+      RCPT TO:<userc@d.bar.org>
+
+   As provided in appendix C, xyz.com MAY also choose to relay the
+   message to hosta.int, using the envelope commands
+
+      MAIL FROM:<userx@y.foo.org>
+      RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org>
+
+   or to jkl.org, using the envelope commands
+
+      MAIL FROM:<userx@y.foo.org>
+      RCPT TO:<@jkl.org:userc@d.bar.org>
+
+   Of course, since hosts are not required to relay mail at all, xyz.com
+   may also reject the message entirely when the RCPT command is
+   received, using a 550 code (since this is a "policy reason").
+
+   If service extensions were negotiated, the RCPT command may also
+   carry parameters associated with a particular service extension
+   offered by the server.  The client MUST NOT transmit parameters other
+   than those associated with a service extension offered by the server
+   in its EHLO response.
+
+Syntax:
+   "RCPT TO:" ("<Postmaster@" domain ">" / "<Postmaster>" / Forward-Path)
+                    [SP Rcpt-parameters] CRLF
+
+
+
+
+
+Klensin                     Standards Track                    [Page 32]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+4.1.1.4 DATA (DATA)
+
+   The receiver normally sends a 354 response to DATA, and then treats
+   the lines (strings ending in <CRLF> sequences, as described in
+   section 2.3.7) following the command as mail data from the sender.
+   This command causes the mail data to be appended to the mail data
+   buffer.  The mail data may contain any of the 128 ASCII character
+   codes, although experience has indicated that use of control
+   characters other than SP, HT, CR, and LF may cause problems and
+   SHOULD be avoided when possible.
+
+   The mail data is terminated by a line containing only a period, that
+   is, the character sequence "<CRLF>.<CRLF>" (see section 4.5.2).  This
+   is the end of mail data indication.  Note that the first <CRLF> of
+   this terminating sequence is also the <CRLF> that ends the final line
+   of the data (message text) or, if there was no data, ends the DATA
+   command itself.  An extra <CRLF> MUST NOT be added, as that would
+   cause an empty line to be added to the message.  The only exception
+   to this rule would arise if the message body were passed to the
+   originating SMTP-sender with a final "line" that did not end in
+   <CRLF>; in that case, the originating SMTP system MUST either reject
+   the message as invalid or add <CRLF> in order to have the receiving
+   SMTP server recognize the "end of data" condition.
+
+   The custom of accepting lines ending only in <LF>, as a concession to
+   non-conforming behavior on the part of some UNIX systems, has proven
+   to cause more interoperability problems than it solves, and SMTP
+   server systems MUST NOT do this, even in the name of improved
+   robustness.  In particular, the sequence "<LF>.<LF>" (bare line
+   feeds, without carriage returns) MUST NOT be treated as equivalent to
+   <CRLF>.<CRLF> as the end of mail data indication.
+
+   Receipt of the end of mail data indication requires the server to
+   process the stored mail transaction information.  This processing
+   consumes the information in the reverse-path buffer, the forward-path
+   buffer, and the mail data buffer, and on the completion of this
+   command these buffers are cleared.  If the processing is successful,
+   the receiver MUST send an OK reply.  If the processing fails the
+   receiver MUST send a failure reply.  The SMTP model does not allow
+   for partial failures at this point: either the message is accepted by
+   the server for delivery and a positive response is returned or it is
+   not accepted and a failure reply is returned.  In sending a positive
+   completion reply to the end of data indication, the receiver takes
+   full responsibility for the message (see section 6.1).  Errors that
+   are diagnosed subsequently MUST be reported in a mail message, as
+   discussed in section 4.4.
+
+
+
+
+
+Klensin                     Standards Track                    [Page 33]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   When the SMTP server accepts a message either for relaying or for
+   final delivery, it inserts a trace record (also referred to
+   interchangeably as a "time stamp line" or "Received" line) at the top
+   of the mail data.  This trace record indicates the identity of the
+   host that sent the message, the identity of the host that received
+   the message (and is inserting this time stamp), and the date and time
+   the message was received.  Relayed messages will have multiple time
+   stamp lines.  Details for formation of these lines, including their
+   syntax, is specified in section 4.4.
+
+   Additional discussion about the operation of the DATA command appears
+   in section 3.3.
+
+   Syntax:
+      "DATA" CRLF
+
+4.1.1.5 RESET (RSET)
+
+   This command specifies that the current mail transaction will be
+   aborted.  Any stored sender, recipients, and mail data MUST be
+   discarded, and all buffers and state tables cleared.  The receiver
+   MUST send a "250 OK" reply to a RSET command with no arguments.  A
+   reset command may be issued by the client at any time.  It is
+   effectively equivalent to a NOOP (i.e., if has no effect) if issued
+   immediately after EHLO, before EHLO is issued in the session, after
+   an end-of-data indicator has been sent and acknowledged, or
+   immediately before a QUIT.  An SMTP server MUST NOT close the
+   connection as the result of receiving a RSET; that action is reserved
+   for QUIT (see section 4.1.1.10).
+
+   Since EHLO implies some additional processing and response by the
+   server, RSET will normally be more efficient than reissuing that
+   command, even though the formal semantics are the same.
+
+   There are circumstances, contrary to the intent of this
+   specification, in which an SMTP server may receive an indication that
+   the underlying TCP connection has been closed or reset.  To preserve
+   the robustness of the mail system, SMTP servers SHOULD be prepared
+   for this condition and SHOULD treat it as if a QUIT had been received
+   before the connection disappeared.
+
+   Syntax:
+      "RSET" CRLF
+
+
+
+
+
+
+
+
+Klensin                     Standards Track                    [Page 34]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+4.1.1.6 VERIFY (VRFY)
+
+   This command asks the receiver to confirm that the argument
+   identifies a user or mailbox.  If it is a user name, information is
+   returned as specified in section 3.5.
+
+   This command has no effect on the reverse-path buffer, the forward-
+   path buffer, or the mail data buffer.
+
+   Syntax:
+      "VRFY" SP String CRLF
+
+4.1.1.7 EXPAND (EXPN)
+
+   This command asks the receiver to confirm that the argument
+   identifies a mailing list, and if so, to return the membership of
+   that list.  If the command is successful, a reply is returned
+   containing information as described in section 3.5.  This reply will
+   have multiple lines except in the trivial case of a one-member list.
+
+   This command has no effect on the reverse-path buffer, the forward-
+   path buffer, or the mail data buffer and may be issued at any time.
+
+   Syntax:
+      "EXPN" SP String CRLF
+
+4.1.1.8 HELP (HELP)
+
+   This command causes the server to send helpful information to the
+   client.  The command MAY take an argument (e.g., any command name)
+   and return more specific information as a response.
+
+   This command has no effect on the reverse-path buffer, the forward-
+   path buffer, or the mail data buffer and may be issued at any time.
+
+   SMTP servers SHOULD support HELP without arguments and MAY support it
+   with arguments.
+
+   Syntax:
+      "HELP" [ SP String ] CRLF
+
+4.1.1.9 NOOP (NOOP)
+
+   This command does not affect any parameters or previously entered
+   commands.  It specifies no action other than that the receiver send
+   an OK reply.
+
+
+
+
+
+Klensin                     Standards Track                    [Page 35]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   This command has no effect on the reverse-path buffer, the forward-
+   path buffer, or the mail data buffer and may be issued at any time.
+   If a parameter string is specified, servers SHOULD ignore it.
+
+   Syntax:
+      "NOOP" [ SP String ] CRLF
+
+4.1.1.10 QUIT (QUIT)
+
+   This command specifies that the receiver MUST send an OK reply, and
+   then close the transmission channel.
+
+   The receiver MUST NOT intentionally close the transmission channel
+   until it receives and replies to a QUIT command (even if there was an
+   error).  The sender MUST NOT intentionally close the transmission
+   channel until it sends a QUIT command and SHOULD wait until it
+   receives the reply (even if there was an error response to a previous
+   command).  If the connection is closed prematurely due to violations
+   of the above or system or network failure, the server MUST cancel any
+   pending transaction, but not undo any previously completed
+   transaction, and generally MUST act as if the command or transaction
+   in progress had received a temporary error (i.e., a 4yz response).
+
+   The QUIT command may be issued at any time.
+
+   Syntax:
+      "QUIT" CRLF
+
+4.1.2 Command Argument Syntax
+
+   The syntax of the argument fields of the above commands (using the
+   syntax specified in [8] where applicable) is given below.  Some of
+   the productions given below are used only in conjunction with source
+   routes as described in appendix C.  Terminals not defined in this
+   document, such as ALPHA, DIGIT, SP, CR, LF, CRLF, are as defined in
+   the "core" syntax [8 (section 6)] or in the message format syntax
+   [32].
+
+      Reverse-path = Path
+      Forward-path = Path
+      Path = "<" [ A-d-l ":" ] Mailbox ">"
+      A-d-l = At-domain *( "," A-d-l )
+            ; Note that this form, the so-called "source route",
+            ; MUST BE accepted, SHOULD NOT be generated, and SHOULD be
+            ; ignored.
+      At-domain = "@" domain
+      Mail-parameters = esmtp-param *(SP esmtp-param)
+      Rcpt-parameters = esmtp-param *(SP esmtp-param)
+
+
+
+Klensin                     Standards Track                    [Page 36]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+      esmtp-param     = esmtp-keyword ["=" esmtp-value]
+      esmtp-keyword   = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")
+      esmtp-value     = 1*(%d33-60 / %d62-127)
+            ; any CHAR excluding "=", SP, and control characters
+      Keyword  = Ldh-str
+      Argument = Atom
+      Domain = (sub-domain 1*("." sub-domain)) / address-literal
+      sub-domain = Let-dig [Ldh-str]
+
+      address-literal = "[" IPv4-address-literal /
+                            IPv6-address-literal /
+                            General-address-literal "]"
+            ; See section 4.1.3
+
+      Mailbox = Local-part "@" Domain
+
+      Local-part = Dot-string / Quoted-string
+            ; MAY be case-sensitive
+
+      Dot-string = Atom *("." Atom)
+
+      Atom = 1*atext
+
+      Quoted-string = DQUOTE *qcontent DQUOTE
+
+      String = Atom / Quoted-string
+
+   While the above definition for Local-part is relatively permissive,
+   for maximum interoperability, a host that expects to receive mail
+   SHOULD avoid defining mailboxes where the Local-part requires (or
+   uses) the Quoted-string form or where the Local-part is case-
+   sensitive.  For any purposes that require generating or comparing
+   Local-parts (e.g., to specific mailbox names), all quoted forms MUST
+   be treated as equivalent and the sending system SHOULD transmit the
+   form that uses the minimum quoting possible.
+
+   Systems MUST NOT define mailboxes in such a way as to require the use
+   in SMTP of non-ASCII characters (octets with the high order bit set
+   to one) or ASCII "control characters" (decimal value 0-31 and 127).
+   These characters MUST NOT be used in MAIL or RCPT commands or other
+   commands that require mailbox names.
+
+   Note that the backslash, "\", is a quote character, which is used to
+   indicate that the next character is to be used literally (instead of
+   its normal interpretation).  For example, "Joe\,Smith" indicates a
+   single nine character user field with the comma being the fourth
+   character of the field.
+
+
+
+
+Klensin                     Standards Track                    [Page 37]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   To promote interoperability and consistent with long-standing
+   guidance about conservative use of the DNS in naming and applications
+   (e.g., see section 2.3.1 of the base DNS document, RFC1035 [22]),
+   characters outside the set of alphas, digits, and hyphen MUST NOT
+   appear in domain name labels for SMTP clients or servers.  In
+   particular, the underscore character is not permitted.  SMTP servers
+   that receive a command in which invalid character codes have been
+   employed, and for which there are no other reasons for rejection,
+   MUST reject that command with a 501 response.
+
+4.1.3 Address Literals
+
+   Sometimes a host is not known to the domain name system and
+   communication (and, in particular, communication to report and repair
+   the error) is blocked.  To bypass this barrier a special literal form
+   of the address is allowed as an alternative to a domain name.  For
+   IPv4 addresses, this form uses four small decimal integers separated
+   by dots and enclosed by brackets such as [123.255.37.2], which
+   indicates an (IPv4) Internet Address in sequence-of-octets form.  For
+   IPv6 and other forms of addressing that might eventually be
+   standardized, the form consists of a standardized "tag" that
+   identifies the address syntax, a colon, and the address itself, in a
+   format specified as part of the IPv6 standards [17].
+
+   Specifically:
+
+      IPv4-address-literal = Snum 3("." Snum)
+      IPv6-address-literal = "IPv6:" IPv6-addr
+      General-address-literal = Standardized-tag ":" 1*dcontent
+      Standardized-tag = Ldh-str
+            ; MUST be specified in a standards-track RFC
+            ; and registered with IANA
+
+      Snum = 1*3DIGIT  ; representing a decimal integer
+            ; value in the range 0 through 255
+      Let-dig = ALPHA / DIGIT
+      Ldh-str = *( ALPHA / DIGIT / "-" ) Let-dig
+
+      IPv6-addr = IPv6-full / IPv6-comp / IPv6v4-full / IPv6v4-comp
+      IPv6-hex  = 1*4HEXDIG
+      IPv6-full = IPv6-hex 7(":" IPv6-hex)
+      IPv6-comp = [IPv6-hex *5(":" IPv6-hex)] "::" [IPv6-hex *5(":"
+                 IPv6-hex)]
+            ; The "::" represents at least 2 16-bit groups of zeros
+            ; No more than 6 groups in addition to the "::" may be
+            ; present
+      IPv6v4-full = IPv6-hex 5(":" IPv6-hex) ":" IPv4-address-literal
+      IPv6v4-comp = [IPv6-hex *3(":" IPv6-hex)] "::"
+
+
+
+Klensin                     Standards Track                    [Page 38]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+                   [IPv6-hex *3(":" IPv6-hex) ":"] IPv4-address-literal
+            ; The "::" represents at least 2 16-bit groups of zeros
+            ; No more than 4 groups in addition to the "::" and
+            ; IPv4-address-literal may be present
+
+4.1.4 Order of Commands
+
+   There are restrictions on the order in which these commands may be
+   used.
+
+   A session that will contain mail transactions MUST first be
+   initialized by the use of the EHLO command.  An SMTP server SHOULD
+   accept commands for non-mail transactions (e.g., VRFY or EXPN)
+   without this initialization.
+
+   An EHLO command MAY be issued by a client later in the session.  If
+   it is issued after the session begins, the SMTP server MUST clear all
+   buffers and reset the state exactly as if a RSET command had been
+   issued.  In other words, the sequence of RSET followed immediately by
+   EHLO is redundant, but not harmful other than in the performance cost
+   of executing unnecessary commands.
+
+   If the EHLO command is not acceptable to the SMTP server, 501, 500,
+   or 502 failure replies MUST be returned as appropriate.  The SMTP
+   server MUST stay in the same state after transmitting these replies
+   that it was in before the EHLO was received.
+
+   The SMTP client MUST, if possible, ensure that the domain parameter
+   to the EHLO command is a valid principal host name (not a CNAME or MX
+   name) for its host.  If this is not possible (e.g., when the client's
+   address is dynamically assigned and the client does not have an
+   obvious name), an address literal SHOULD be substituted for the
+   domain name and supplemental information provided that will assist in
+   identifying the client.
+
+   An SMTP server MAY verify that the domain name parameter in the EHLO
+   command actually corresponds to the IP address of the client.
+   However, the server MUST NOT refuse to accept a message for this
+   reason if the verification fails: the information about verification
+   failure is for logging and tracing only.
+
+   The NOOP, HELP, EXPN, VRFY, and RSET commands can be used at any time
+   during a session, or without previously initializing a session.  SMTP
+   servers SHOULD process these normally (that is, not return a 503
+   code) even if no EHLO command has yet been received; clients SHOULD
+   open a session with EHLO before sending these commands.
+
+
+
+
+
+Klensin                     Standards Track                    [Page 39]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   If these rules are followed, the example in RFC 821 that shows "550
+   access denied to you" in response to an EXPN command is incorrect
+   unless an EHLO command precedes the EXPN or the denial of access is
+   based on the client's IP address or other authentication or
+   authorization-determining mechanisms.
+
+   The MAIL command (or the obsolete SEND, SOML, or SAML commands)
+   begins a mail transaction.  Once started, a mail transaction consists
+   of a transaction beginning command, one or more RCPT commands, and a
+   DATA command, in that order.  A mail transaction may be aborted by
+   the RSET (or a new EHLO) command.  There may be zero or more
+   transactions in a session.  MAIL (or SEND, SOML, or SAML) MUST NOT be
+   sent if a mail transaction is already open, i.e., it should be sent
+   only if no mail transaction had been started in the session, or it
+   the previous one successfully concluded with a successful DATA
+   command, or if the previous one was aborted with a RSET.
+
+   If the transaction beginning command argument is not acceptable, a
+   501 failure reply MUST be returned and the SMTP server MUST stay in
+   the same state.  If the commands in a transaction are out of order to
+   the degree that they cannot be processed by the server, a 503 failure
+   reply MUST be returned and the SMTP server MUST stay in the same
+   state.
+
+   The last command in a session MUST be the QUIT command.  The QUIT
+   command cannot be used at any other time in a session, but SHOULD be
+   used by the client SMTP to request connection closure, even when no
+   session opening command was sent and accepted.
+
+4.1.5 Private-use Commands
+
+   As specified in section 2.2.2, commands starting in "X" may be used
+   by bilateral agreement between the client (sending) and server
+   (receiving) SMTP agents.  An SMTP server that does not recognize such
+   a command is expected to reply with "500 Command not recognized".  An
+   extended SMTP server MAY list the feature names associated with these
+   private commands in the response to the EHLO command.
+
+   Commands sent or accepted by SMTP systems that do not start with "X"
+   MUST conform to the requirements of section 2.2.2.
+
+4.2 SMTP Replies
+
+   Replies to SMTP commands serve to ensure the synchronization of
+   requests and actions in the process of mail transfer and to guarantee
+   that the SMTP client always knows the state of the SMTP server.
+   Every command MUST generate exactly one reply.
+
+
+
+
+Klensin                     Standards Track                    [Page 40]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   The details of the command-reply sequence are described in section
+   4.3.
+
+   An SMTP reply consists of a three digit number (transmitted as three
+   numeric characters) followed by some text unless specified otherwise
+   in this document.  The number is for use by automata to determine
+   what state to enter next; the text is for the human user.  The three
+   digits contain enough encoded information that the SMTP client need
+   not examine the text and may either discard it or pass it on to the
+   user, as appropriate.  Exceptions are as noted elsewhere in this
+   document.  In particular, the 220, 221, 251, 421, and 551 reply codes
+   are associated with message text that must be parsed and interpreted
+   by machines.  In the general case, the text may be receiver dependent
+   and context dependent, so there are likely to be varying texts for
+   each reply code.  A discussion of the theory of reply codes is given
+   in section 4.2.1.  Formally, a reply is defined to be the sequence: a
+   three-digit code, <SP>, one line of text, and <CRLF>, or a multiline
+   reply (as defined in section 4.2.1).  Since, in violation of this
+   specification, the text is sometimes not sent, clients which do not
+   receive it SHOULD be prepared to process the code alone (with or
+   without a trailing space character).  Only the EHLO, EXPN, and HELP
+   commands are expected to result in multiline replies in normal
+   circumstances, however, multiline replies are allowed for any
+   command.
+
+   In ABNF, server responses are:
+
+      Greeting = "220 " Domain [ SP text ] CRLF
+      Reply-line = Reply-code [ SP text ] CRLF
+
+   where "Greeting" appears only in the 220 response that announces that
+   the server is opening its part of the connection.
+
+   An SMTP server SHOULD send only the reply codes listed in this
+   document.  An SMTP server SHOULD use the text shown in the examples
+   whenever appropriate.
+
+   An SMTP client MUST determine its actions only by the reply code, not
+   by the text (except for the "change of address" 251 and 551 and, if
+   necessary, 220, 221, and 421 replies); in the general case, any text,
+   including no text at all (although senders SHOULD NOT send bare
+   codes), MUST be acceptable.  The space (blank) following the reply
+   code is considered part of the text.  Whenever possible, a receiver-
+   SMTP SHOULD test the first digit (severity indication) of the reply
+   code.
+
+
+
+
+
+
+Klensin                     Standards Track                    [Page 41]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   The list of codes that appears below MUST NOT be construed as
+   permanent.  While the addition of new codes should be a rare and
+   significant activity, with supplemental information in the textual
+   part of the response being preferred, new codes may be added as the
+   result of new Standards or Standards-track specifications.
+   Consequently, a sender-SMTP MUST be prepared to handle codes not
+   specified in this document and MUST do so by interpreting the first
+   digit only.
+
+4.2.1 Reply Code Severities and Theory
+
+   The three digits of the reply each have a special significance.  The
+   first digit denotes whether the response is good, bad or incomplete.
+   An unsophisticated SMTP client, or one that receives an unexpected
+   code, will be able to determine its next action (proceed as planned,
+   redo, retrench, etc.) by examining this first digit.  An SMTP client
+   that wants to know approximately what kind of error occurred (e.g.,
+   mail system error, command syntax error) may examine the second
+   digit.  The third digit and any supplemental information that may be
+   present is reserved for the finest gradation of information.
+
+   There are five values for the first digit of the reply code:
+
+   1yz   Positive Preliminary reply
+      The command has been accepted, but the requested action is being
+      held in abeyance, pending confirmation of the information in this
+      reply.  The SMTP client should send another command specifying
+      whether to continue or abort the action.  Note: unextended SMTP
+      does not have any commands that allow this type of reply, and so
+      does not have continue or abort commands.
+
+   2yz   Positive Completion reply
+      The requested action has been successfully completed.  A new
+      request may be initiated.
+
+   3yz   Positive Intermediate reply
+      The command has been accepted, but the requested action is being
+      held in abeyance, pending receipt of further information.  The
+      SMTP client should send another command specifying this
+      information.  This reply is used in command sequence groups (i.e.,
+      in DATA).
+
+   4yz   Transient Negative Completion reply
+      The command was not accepted, and the requested action did not
+      occur.  However, the error condition is temporary and the action
+      may be requested again.  The sender should return to the beginning
+      of the command sequence (if any).  It is difficult to assign a
+      meaning to "transient" when two different sites (receiver- and
+
+
+
+Klensin                     Standards Track                    [Page 42]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+      sender-SMTP agents) must agree on the interpretation.  Each reply
+      in this category might have a different time value, but the SMTP
+      client is encouraged to try again.  A rule of thumb to determine
+      whether a reply fits into the 4yz or the 5yz category (see below)
+      is that replies are 4yz if they can be successful if repeated
+      without any change in command form or in properties of the sender
+      or receiver (that is, the command is repeated identically and the
+      receiver does not put up a new implementation.)
+
+   5yz   Permanent Negative Completion reply
+      The command was not accepted and the requested action did not
+      occur.  The SMTP client is discouraged from repeating the exact
+      request (in the same sequence).  Even some "permanent" error
+      conditions can be corrected, so the human user may want to direct
+      the SMTP client to reinitiate the command sequence by direct
+      action at some point in the future (e.g., after the spelling has
+      been changed, or the user has altered the account status).
+
+   The second digit encodes responses in specific categories:
+
+   x0z   Syntax: These replies refer to syntax errors, syntactically
+      correct commands that do not fit any functional category, and
+      unimplemented or superfluous commands.
+
+   x1z   Information:  These are replies to requests for information,
+      such as status or help.
+
+   x2z   Connections: These are replies referring to the transmission
+      channel.
+
+   x3z   Unspecified.
+
+   x4z   Unspecified.
+
+   x5z   Mail system: These replies indicate the status of the receiver
+      mail system vis-a-vis the requested transfer or other mail system
+      action.
+
+   The third digit gives a finer gradation of meaning in each category
+   specified by the second digit.  The list of replies illustrates this.
+   Each reply text is recommended rather than mandatory, and may even
+   change according to the command with which it is associated.  On the
+   other hand, the reply codes must strictly follow the specifications
+   in this section.  Receiver implementations should not invent new
+   codes for slightly different situations from the ones described here,
+   but rather adapt codes already defined.
+
+
+
+
+
+Klensin                     Standards Track                    [Page 43]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   For example, a command such as NOOP, whose successful execution does
+   not offer the SMTP client any new information, will return a 250
+   reply.  The reply is 502 when the command requests an unimplemented
+   non-site-specific action.  A refinement of that is the 504 reply for
+   a command that is implemented, but that requests an unimplemented
+   parameter.
+
+   The reply text may be longer than a single line; in these cases the
+   complete text must be marked so the SMTP client knows when it can
+   stop reading the reply.  This requires a special format to indicate a
+   multiple line reply.
+
+   The format for multiline replies requires that every line, except the
+   last, begin with the reply code, followed immediately by a hyphen,
+   "-" (also known as minus), followed by text.  The last line will
+   begin with the reply code, followed immediately by <SP>, optionally
+   some text, and <CRLF>.  As noted above, servers SHOULD send the <SP>
+   if subsequent text is not sent, but clients MUST be prepared for it
+   to be omitted.
+
+   For example:
+
+      123-First line
+      123-Second line
+      123-234 text beginning with numbers
+      123 The last line
+
+   In many cases the SMTP client then simply needs to search for a line
+   beginning with the reply code followed by <SP> or <CRLF> and ignore
+   all preceding lines.  In a few cases, there is important data for the
+   client in the reply "text".  The client will be able to identify
+   these cases from the current context.
+
+4.2.2 Reply Codes by Function Groups
+
+      500 Syntax error, command unrecognized
+         (This may include errors such as command line too long)
+      501 Syntax error in parameters or arguments
+      502 Command not implemented  (see section 4.2.4)
+      503 Bad sequence of commands
+      504 Command parameter not implemented
+
+      211 System status, or system help reply
+      214 Help message
+         (Information on how to use the receiver or the meaning of a
+         particular non-standard command; this reply is useful only
+         to the human user)
+
+
+
+
+Klensin                     Standards Track                    [Page 44]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+      220 <domain> Service ready
+      221 <domain> Service closing transmission channel
+      421 <domain> Service not available, closing transmission channel
+         (This may be a reply to any command if the service knows it
+         must shut down)
+
+      250 Requested mail action okay, completed
+      251 User not local; will forward to <forward-path>
+         (See section 3.4)
+      252 Cannot VRFY user, but will accept message and attempt
+          delivery
+         (See section 3.5.3)
+      450 Requested mail action not taken: mailbox unavailable
+         (e.g., mailbox busy)
+      550 Requested action not taken: mailbox unavailable
+         (e.g., mailbox not found, no access, or command rejected
+         for policy reasons)
+      451 Requested action aborted: error in processing
+      551 User not local; please try <forward-path>
+         (See section 3.4)
+      452 Requested action not taken: insufficient system storage
+      552 Requested mail action aborted: exceeded storage allocation
+      553 Requested action not taken: mailbox name not allowed
+         (e.g., mailbox syntax incorrect)
+      354 Start mail input; end with <CRLF>.<CRLF>
+      554 Transaction failed (Or, in the case of a connection-opening
+          response, "No SMTP service here")
+
+4.2.3  Reply Codes in Numeric Order
+
+      211 System status, or system help reply
+      214 Help message
+         (Information on how to use the receiver or the meaning of a
+         particular non-standard command; this reply is useful only
+         to the human user)
+      220 <domain> Service ready
+      221 <domain> Service closing transmission channel
+      250 Requested mail action okay, completed
+      251 User not local; will forward to <forward-path>
+         (See section 3.4)
+      252 Cannot VRFY user, but will accept message and attempt
+         delivery
+         (See section 3.5.3)
+
+      354 Start mail input; end with <CRLF>.<CRLF>
+
+
+
+
+
+
+Klensin                     Standards Track                    [Page 45]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+      421 <domain> Service not available, closing transmission channel
+         (This may be a reply to any command if the service knows it
+         must shut down)
+      450 Requested mail action not taken: mailbox unavailable
+         (e.g., mailbox busy)
+      451 Requested action aborted: local error in processing
+      452 Requested action not taken: insufficient system storage
+      500 Syntax error, command unrecognized
+         (This may include errors such as command line too long)
+      501 Syntax error in parameters or arguments
+      502 Command not implemented (see section 4.2.4)
+      503 Bad sequence of commands
+      504 Command parameter not implemented
+      550 Requested action not taken: mailbox unavailable
+         (e.g., mailbox not found, no access, or command rejected
+         for policy reasons)
+      551 User not local; please try <forward-path>
+         (See section 3.4)
+      552 Requested mail action aborted: exceeded storage allocation
+      553 Requested action not taken: mailbox name not allowed
+         (e.g., mailbox syntax incorrect)
+      554 Transaction failed  (Or, in the case of a connection-opening
+          response, "No SMTP service here")
+
+4.2.4 Reply Code 502
+
+   Questions have been raised as to when reply code 502 (Command not
+   implemented) SHOULD be returned in preference to other codes.  502
+   SHOULD be used when the command is actually recognized by the SMTP
+   server, but not implemented.  If the command is not recognized, code
+   500 SHOULD be returned.  Extended SMTP systems MUST NOT list
+   capabilities in response to EHLO for which they will return 502 (or
+   500) replies.
+
+4.2.5 Reply Codes After DATA and the Subsequent <CRLF>.<CRLF>
+
+   When an SMTP server returns a positive completion status (2yz code)
+   after the DATA command is completed with <CRLF>.<CRLF>, it accepts
+   responsibility for:
+
+   -  delivering the message (if the recipient mailbox exists), or
+
+   -  if attempts to deliver the message fail due to transient
+      conditions, retrying delivery some reasonable number of times at
+      intervals as specified in section 4.5.4.
+
+
+
+
+
+
+Klensin                     Standards Track                    [Page 46]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   -  if attempts to deliver the message fail due to permanent
+      conditions, or if repeated attempts to deliver the message fail
+      due to transient conditions, returning appropriate notification to
+      the sender of the original message (using the address in the SMTP
+      MAIL command).
+
+   When an SMTP server returns a permanent error status (5yz) code after
+   the DATA command is completed with <CRLF>.<CRLF>, it MUST NOT make
+   any subsequent attempt to deliver that message.  The SMTP client
+   retains responsibility for delivery of that message and may either
+   return it to the user or requeue it for a subsequent attempt (see
+   section 4.5.4.1).
+
+   The user who originated the message SHOULD be able to interpret the
+   return of a transient failure status (by mail message or otherwise)
+   as a non-delivery indication, just as a permanent failure would be
+   interpreted.  I.e., if the client SMTP successfully handles these
+   conditions, the user will not receive such a reply.
+
+   When an SMTP server returns a permanent error status (5yz) code after
+   the DATA command is completely with <CRLF>.<CRLF>, it MUST NOT make
+   any subsequent attempt to deliver the message.  As with temporary
+   error status codes, the SMTP client retains responsibility for the
+   message, but SHOULD not again attempt delivery to the same server
+   without user review and intervention of the message.
+
+4.3 Sequencing of Commands and Replies
+
+4.3.1 Sequencing Overview
+
+   The communication between the sender and receiver is an alternating
+   dialogue, controlled by the sender.  As such, the sender issues a
+   command and the receiver responds with a reply.  Unless other
+   arrangements are negotiated through service extensions, the sender
+   MUST wait for this response before sending further commands.
+
+   One important reply is the connection greeting.  Normally, a receiver
+   will send a 220 "Service ready" reply when the connection is
+   completed.  The sender SHOULD wait for this greeting message before
+   sending any commands.
+
+   Note: all the greeting-type replies have the official name (the
+   fully-qualified primary domain name) of the server host as the first
+   word following the reply code.  Sometimes the host will have no
+   meaningful name.  See 4.1.3 for a discussion of alternatives in these
+   situations.
+
+
+
+
+
+Klensin                     Standards Track                    [Page 47]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   For example,
+
+      220 ISIF.USC.EDU Service ready
+   or
+      220 mail.foo.com SuperSMTP v 6.1.2 Service ready
+   or
+      220 [10.0.0.1] Clueless host service ready
+
+   The table below lists alternative success and failure replies for
+   each command.  These SHOULD be strictly adhered to: a receiver may
+   substitute text in the replies, but the meaning and action implied by
+   the code numbers and by the specific command reply sequence cannot be
+   altered.
+
+4.3.2 Command-Reply Sequences
+
+   Each command is listed with its usual possible replies.  The prefixes
+   used before the possible replies are "I" for intermediate, "S" for
+   success, and "E" for error.  Since some servers may generate other
+   replies under special circumstances, and to allow for future
+   extension, SMTP clients SHOULD, when possible, interpret only the
+   first digit of the reply and MUST be prepared to deal with
+   unrecognized reply codes by interpreting the first digit only.
+   Unless extended using the mechanisms described in section 2.2, SMTP
+   servers MUST NOT transmit reply codes to an SMTP client that are
+   other than three digits or that do not start in a digit between 2 and
+   5 inclusive.
+
+   These sequencing rules and, in principle, the codes themselves, can
+   be extended or modified by SMTP extensions offered by the server and
+   accepted (requested) by the client.
+
+   In addition to the codes listed below, any SMTP command can return
+   any of the following codes if the corresponding unusual circumstances
+   are encountered:
+
+   500  For the "command line too long" case or if the command name was
+      not recognized.  Note that producing a "command not recognized"
+      error in response to the required subset of these commands is a
+      violation of this specification.
+
+   501  Syntax error in command or arguments.  In order to provide for
+      future extensions, commands that are specified in this document as
+      not accepting arguments (DATA, RSET, QUIT) SHOULD return a 501
+      message if arguments are supplied in the absence of EHLO-
+      advertised extensions.
+
+   421  Service shutting down and closing transmission channel
+
+
+
+Klensin                     Standards Track                    [Page 48]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   Specific sequences are:
+
+   CONNECTION ESTABLISHMENT
+      S: 220
+      E: 554
+   EHLO or HELO
+      S: 250
+      E: 504, 550
+   MAIL
+      S: 250
+      E: 552, 451, 452, 550, 553, 503
+   RCPT
+      S: 250, 251 (but see section 3.4 for discussion of 251 and 551)
+      E: 550, 551, 552, 553, 450, 451, 452, 503, 550
+   DATA
+      I: 354 -> data -> S: 250
+                        E: 552, 554, 451, 452
+      E: 451, 554, 503
+   RSET
+      S: 250
+   VRFY
+      S: 250, 251, 252
+      E: 550, 551, 553, 502, 504
+   EXPN
+      S: 250, 252
+      E: 550, 500, 502, 504
+   HELP
+      S: 211, 214
+      E: 502, 504
+   NOOP
+      S: 250
+   QUIT
+      S: 221
+
+4.4 Trace Information
+
+   When an SMTP server receives a message for delivery or further
+   processing, it MUST insert trace ("time stamp" or "Received")
+   information at the beginning of the message content, as discussed in
+   section 4.1.1.4.
+
+   This line MUST be structured as follows:
+
+   -  The FROM field, which MUST be supplied in an SMTP environment,
+      SHOULD contain both (1) the name of the source host as presented
+      in the EHLO command and (2) an address literal containing the IP
+      address of the source, determined from the TCP connection.
+
+
+
+
+Klensin                     Standards Track                    [Page 49]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   -  The ID field MAY contain an "@" as suggested in RFC 822, but this
+      is not required.
+
+   -  The FOR field MAY contain a list of <path> entries when multiple
+      RCPT commands have been given.  This may raise some security
+      issues and is usually not desirable; see section 7.2.
+
+   An Internet mail program MUST NOT change a Received: line that was
+   previously added to the message header.  SMTP servers MUST prepend
+   Received lines to messages; they MUST NOT change the order of
+   existing lines or insert Received lines in any other location.
+
+   As the Internet grows, comparability of Received fields is important
+   for detecting problems, especially slow relays.  SMTP servers that
+   create Received fields SHOULD use explicit offsets in the dates
+   (e.g., -0800), rather than time zone names of any type.  Local time
+   (with an offset) is preferred to UT when feasible.  This formulation
+   allows slightly more information about local circumstances to be
+   specified.  If UT is needed, the receiver need merely do some simple
+   arithmetic to convert the values.  Use of UT loses information about
+   the time zone-location of the server.  If it is desired to supply a
+   time zone name, it SHOULD be included in a comment.
+
+   When the delivery SMTP server makes the "final delivery" of a
+   message, it inserts a return-path line at the beginning of the mail
+   data.  This use of return-path is required; mail systems MUST support
+   it.  The return-path line preserves the information in the <reverse-
+   path> from the MAIL command.  Here, final delivery means the message
+   has left the SMTP environment.  Normally, this would mean it had been
+   delivered to the destination user or an associated mail drop, but in
+   some cases it may be further processed and transmitted by another
+   mail system.
+
+   It is possible for the mailbox in the return path to be different
+   from the actual sender's mailbox, for example, if error responses are
+   to be delivered to a special error handling mailbox rather than to
+   the message sender.  When mailing lists are involved, this
+   arrangement is common and useful as a means of directing errors to
+   the list maintainer rather than the message originator.
+
+   The text above implies that the final mail data will begin with a
+   return path line, followed by one or more time stamp lines.  These
+   lines will be followed by the mail data headers and body [32].
+
+   It is sometimes difficult for an SMTP server to determine whether or
+   not it is making final delivery since forwarding or other operations
+   may occur after the message is accepted for delivery.  Consequently,
+
+
+
+
+Klensin                     Standards Track                    [Page 50]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   any further (forwarding, gateway, or relay) systems MAY remove the
+   return path and rebuild the MAIL command as needed to ensure that
+   exactly one such line appears in a delivered message.
+
+   A message-originating SMTP system SHOULD NOT send a message that
+   already contains a Return-path header.  SMTP servers performing a
+   relay function MUST NOT inspect the message data, and especially not
+   to the extent needed to determine if Return-path headers are present.
+   SMTP servers making final delivery MAY remove Return-path headers
+   before adding their own.
+
+   The primary purpose of the Return-path is to designate the address to
+   which messages indicating non-delivery or other mail system failures
+   are to be sent.  For this to be unambiguous, exactly one return path
+   SHOULD be present when the message is delivered.  Systems using RFC
+   822 syntax with non-SMTP transports SHOULD designate an unambiguous
+   address, associated with the transport envelope, to which error
+   reports (e.g., non-delivery messages) should be sent.
+
+   Historical note: Text in RFC 822 that appears to contradict the use
+   of the Return-path header (or the envelope reverse path address from
+   the MAIL command) as the destination for error messages is not
+   applicable on the Internet.  The reverse path address (as copied into
+   the Return-path) MUST be used as the target of any mail containing
+   delivery error messages.
+
+   In particular:
+
+   -  a gateway from SMTP->elsewhere SHOULD insert a return-path header,
+      unless it is known that the "elsewhere" transport also uses
+      Internet domain addresses and maintains the envelope sender
+      address separately.
+
+   -  a gateway from elsewhere->SMTP SHOULD delete any return-path
+      header present in the message, and either copy that information to
+      the SMTP envelope or combine it with information present in the
+      envelope of the other transport system to construct the reverse
+      path argument to the MAIL command in the SMTP envelope.
+
+   The server must give special treatment to cases in which the
+   processing following the end of mail data indication is only
+   partially successful.  This could happen if, after accepting several
+   recipients and the mail data, the SMTP server finds that the mail
+   data could be successfully delivered to some, but not all, of the
+   recipients.  In such cases, the response to the DATA command MUST be
+   an OK reply.  However, the SMTP server MUST compose and send an
+   "undeliverable mail" notification message to the originator of the
+   message.
+
+
+
+Klensin                     Standards Track                    [Page 51]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   A single notification listing all of the failed recipients or
+   separate notification messages MUST be sent for each failed
+   recipient.  For economy of processing by the sender, the former is
+   preferred when possible.  All undeliverable mail notification
+   messages are sent using the MAIL command (even if they result from
+   processing the obsolete SEND, SOML, or SAML commands) and use a null
+   return path as discussed in section 3.7.
+
+   The time stamp line and the return path line are formally defined as
+   follows:
+
+Return-path-line = "Return-Path:" FWS Reverse-path <CRLF>
+
+Time-stamp-line = "Received:" FWS Stamp <CRLF>
+
+Stamp = From-domain By-domain Opt-info ";"  FWS date-time
+
+      ; where "date-time" is as defined in [32]
+      ; but the "obs-" forms, especially two-digit
+      ; years, are prohibited in SMTP and MUST NOT be used.
+
+From-domain = "FROM" FWS Extended-Domain CFWS
+
+By-domain = "BY" FWS Extended-Domain CFWS
+
+Extended-Domain = Domain /
+           ( Domain FWS "(" TCP-info ")" ) /
+           ( Address-literal FWS "(" TCP-info ")" )
+
+TCP-info = Address-literal / ( Domain FWS Address-literal )
+      ; Information derived by server from TCP connection
+      ; not client EHLO.
+
+Opt-info = [Via] [With] [ID] [For]
+
+Via = "VIA" FWS Link CFWS
+
+With = "WITH" FWS Protocol CFWS
+
+ID = "ID" FWS String / msg-id CFWS
+
+For = "FOR" FWS 1*( Path / Mailbox ) CFWS
+
+Link = "TCP" / Addtl-Link
+Addtl-Link = Atom
+      ; Additional standard names for links are registered with the
+         ; Internet Assigned Numbers Authority (IANA).  "Via" is
+         ; primarily of value with non-Internet transports.  SMTP
+
+
+
+Klensin                     Standards Track                    [Page 52]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+         ; servers SHOULD NOT use unregistered names.
+Protocol = "ESMTP" / "SMTP" / Attdl-Protocol
+Attdl-Protocol = Atom
+      ; Additional standard names for protocols are registered with the
+         ; Internet Assigned Numbers Authority (IANA).  SMTP servers
+         ; SHOULD NOT use unregistered names.
+
+4.5 Additional Implementation Issues
+
+4.5.1 Minimum Implementation
+
+   In order to make SMTP workable, the following minimum implementation
+   is required for all receivers.  The following commands MUST be
+   supported to conform to this specification:
+
+      EHLO
+      HELO
+      MAIL
+      RCPT
+      DATA
+      RSET
+      NOOP
+      QUIT
+      VRFY
+
+   Any system that includes an SMTP server supporting mail relaying or
+   delivery MUST support the reserved mailbox "postmaster" as a case-
+   insensitive local name.  This postmaster address is not strictly
+   necessary if the server always returns 554 on connection opening (as
+   described in section 3.1).  The requirement to accept mail for
+   postmaster implies that RCPT commands which specify a mailbox for
+   postmaster at any of the domains for which the SMTP server provides
+   mail service, as well as the special case of "RCPT TO:<Postmaster>"
+   (with no domain specification), MUST be supported.
+
+   SMTP systems are expected to make every reasonable effort to accept
+   mail directed to Postmaster from any other system on the Internet.
+   In extreme cases --such as to contain a denial of service attack or
+   other breach of security-- an SMTP server may block mail directed to
+   Postmaster.  However, such arrangements SHOULD be narrowly tailored
+   so as to avoid blocking messages which are not part of such attacks.
+
+4.5.2 Transparency
+
+   Without some provision for data transparency, the character sequence
+   "<CRLF>.<CRLF>" ends the mail text and cannot be sent by the user.
+   In general, users are not aware of such "forbidden" sequences.  To
+
+
+
+
+Klensin                     Standards Track                    [Page 53]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   allow all user composed text to be transmitted transparently, the
+   following procedures are used:
+
+   -  Before sending a line of mail text, the SMTP client checks the
+      first character of the line.  If it is a period, one additional
+      period is inserted at the beginning of the line.
+
+   -  When a line of mail text is received by the SMTP server, it checks
+      the line.  If the line is composed of a single period, it is
+      treated as the end of mail indicator.  If the first character is a
+      period and there are other characters on the line, the first
+      character is deleted.
+
+   The mail data may contain any of the 128 ASCII characters.  All
+   characters are to be delivered to the recipient's mailbox, including
+   spaces, vertical and horizontal tabs, and other control characters.
+   If the transmission channel provides an 8-bit byte (octet) data
+   stream, the 7-bit ASCII codes are transmitted right justified in the
+   octets, with the high order bits cleared to zero.  See 3.7 for
+   special treatment of these conditions in SMTP systems serving a relay
+   function.
+
+   In some systems it may be necessary to transform the data as it is
+   received and stored.  This may be necessary for hosts that use a
+   different character set than ASCII as their local character set, that
+   store data in records rather than strings, or which use special
+   character sequences as delimiters inside mailboxes.  If such
+   transformations are necessary, they MUST be reversible, especially if
+   they are applied to mail being relayed.
+
+4.5.3 Sizes and Timeouts
+
+4.5.3.1 Size limits and minimums
+
+   There are several objects that have required minimum/maximum sizes.
+   Every implementation MUST be able to receive objects of at least
+   these sizes.  Objects larger than these sizes SHOULD be avoided when
+   possible.  However, some Internet mail constructs such as encoded
+   X.400 addresses [16] will often require larger objects: clients MAY
+   attempt to transmit these, but MUST be prepared for a server to
+   reject them if they cannot be handled by it.  To the maximum extent
+   possible, implementation techniques which impose no limits on the
+   length of these objects should be used.
+
+   local-part
+      The maximum total length of a user name or other local-part is 64
+      characters.
+
+
+
+
+Klensin                     Standards Track                    [Page 54]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   domain
+      The maximum total length of a domain name or number is 255
+      characters.
+
+   path
+      The maximum total length of a reverse-path or forward-path is 256
+      characters (including the punctuation and element separators).
+
+   command line
+      The maximum total length of a command line including the command
+      word and the <CRLF> is 512 characters.  SMTP extensions may be
+      used to increase this limit.
+
+   reply line
+      The maximum total length of a reply line including the reply code
+      and the <CRLF> is 512 characters.  More information may be
+      conveyed through multiple-line replies.
+
+   text line
+      The maximum total length of a text line including the <CRLF> is
+      1000 characters (not counting the leading dot duplicated for
+      transparency).  This number may be increased by the use of SMTP
+      Service Extensions.
+
+   message content
+      The maximum total length of a message content (including any
+      message headers as well as the message body) MUST BE at least 64K
+      octets.  Since the introduction of Internet standards for
+      multimedia mail [12], message lengths on the Internet have grown
+      dramatically, and message size restrictions should be avoided if
+      at all possible.  SMTP server systems that must impose
+      restrictions SHOULD implement the "SIZE" service extension [18],
+      and SMTP client systems that will send large messages SHOULD
+      utilize it when possible.
+
+   recipients buffer
+      The minimum total number of recipients that must be buffered is
+      100 recipients.  Rejection of messages (for excessive recipients)
+      with fewer than 100 RCPT commands is a violation of this
+      specification.  The general principle that relaying SMTP servers
+      MUST NOT, and delivery SMTP servers SHOULD NOT, perform validation
+      tests on message headers suggests that rejecting a message based
+      on the total number of recipients shown in header fields is to be
+      discouraged.  A server which imposes a limit on the number of
+      recipients MUST behave in an orderly fashion,  such as to reject
+      additional addresses over its limit rather than silently
+      discarding addresses previously accepted.  A client that needs to
+
+
+
+
+Klensin                     Standards Track                    [Page 55]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+      deliver a message containing over 100 RCPT commands SHOULD be
+      prepared to transmit in 100-recipient "chunks" if the server
+      declines to accept more than 100 recipients in a single message.
+
+   Errors due to exceeding these limits may be reported by using the
+   reply codes.  Some examples of reply codes are:
+
+      500 Line too long.
+   or
+      501 Path too long
+   or
+      452 Too many recipients  (see below)
+   or
+      552 Too much mail data.
+
+   RFC 821 [30] incorrectly listed the error where an SMTP server
+   exhausts its implementation limit on the number of RCPT commands
+   ("too many recipients") as having reply code 552.  The correct reply
+   code for this condition is 452.  Clients SHOULD treat a 552 code in
+   this case as a temporary, rather than permanent, failure so the logic
+   below works.
+
+   When a conforming SMTP server encounters this condition, it has at
+   least 100 successful RCPT commands in its recipients buffer.  If the
+   server is able to accept the message, then at least these 100
+   addresses will be removed from the SMTP client's queue.  When the
+   client attempts retransmission of those addresses which received 452
+   responses, at least 100 of these will be able to fit in the SMTP
+   server's recipients buffer.  Each retransmission attempt which is
+   able to deliver anything will be able to dispose of at least 100 of
+   these recipients.
+
+   If an SMTP server has an implementation limit on the number of RCPT
+   commands and this limit is exhausted, it MUST use a response code of
+   452 (but the client SHOULD also be prepared for a 552, as noted
+   above).  If the server has a configured site-policy limitation on the
+   number of RCPT commands, it MAY instead use a 5XX response code.
+   This would be most appropriate if the policy limitation was intended
+   to apply if the total recipient count for a particular message body
+   were enforced even if that message body was sent in multiple mail
+   transactions.
+
+4.5.3.2 Timeouts
+
+   An SMTP client MUST provide a timeout mechanism.  It MUST use per-
+   command timeouts rather than somehow trying to time the entire mail
+   transaction.  Timeouts SHOULD be easily reconfigurable, preferably
+   without recompiling the SMTP code.  To implement this, a timer is set
+
+
+
+Klensin                     Standards Track                    [Page 56]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   for each SMTP command and for each buffer of the data transfer.  The
+   latter means that the overall timeout is inherently proportional to
+   the size of the message.
+
+   Based on extensive experience with busy mail-relay hosts, the minimum
+   per-command timeout values SHOULD be as follows:
+
+   Initial 220 Message: 5 minutes
+      An SMTP client process needs to distinguish between a failed TCP
+      connection and a delay in receiving the initial 220 greeting
+      message.  Many SMTP servers accept a TCP connection but delay
+      delivery of the 220 message until their system load permits more
+      mail to be processed.
+
+   MAIL Command: 5 minutes
+
+   RCPT Command: 5 minutes
+      A longer timeout is required if processing of mailing lists and
+      aliases is not deferred until after the message was accepted.
+
+   DATA Initiation: 2 minutes
+      This is while awaiting the "354 Start Input" reply to a DATA
+      command.
+
+   Data Block: 3 minutes
+      This is while awaiting the completion of each TCP SEND call
+      transmitting a chunk of data.
+
+   DATA Termination: 10 minutes.
+      This is while awaiting the "250 OK" reply.  When the receiver gets
+      the final period terminating the message data, it typically
+      performs processing to deliver the message to a user mailbox.  A
+      spurious timeout at this point would be very wasteful and would
+      typically result in delivery of multiple copies of the message,
+      since it has been successfully sent and the server has accepted
+      responsibility for delivery.  See section 6.1 for additional
+      discussion.
+
+   An SMTP server SHOULD have a timeout of at least 5 minutes while it
+   is awaiting the next command from the sender.
+
+4.5.4 Retry Strategies
+
+   The common structure of a host SMTP implementation includes user
+   mailboxes, one or more areas for queuing messages in transit, and one
+   or more daemon processes for sending and receiving mail.  The exact
+   structure will vary depending on the needs of the users on the host
+
+
+
+
+Klensin                     Standards Track                    [Page 57]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   and the number and size of mailing lists supported by the host.  We
+   describe several optimizations that have proved helpful, particularly
+   for mailers supporting high traffic levels.
+
+   Any queuing strategy MUST include timeouts on all activities on a
+   per-command basis.  A queuing strategy MUST NOT send error messages
+   in response to error messages under any circumstances.
+
+4.5.4.1 Sending Strategy
+
+   The general model for an SMTP client is one or more processes that
+   periodically attempt to transmit outgoing mail.  In a typical system,
+   the program that composes a message has some method for requesting
+   immediate attention for a new piece of outgoing mail, while mail that
+   cannot be transmitted immediately MUST be queued and periodically
+   retried by the sender.  A mail queue entry will include not only the
+   message itself but also the envelope information.
+
+   The sender MUST delay retrying a particular destination after one
+   attempt has failed.  In general, the retry interval SHOULD be at
+   least 30 minutes; however, more sophisticated and variable strategies
+   will be beneficial when the SMTP client can determine the reason for
+   non-delivery.
+
+   Retries continue until the message is transmitted or the sender gives
+   up; the give-up time generally needs to be at least 4-5 days.  The
+   parameters to the retry algorithm MUST be configurable.
+
+   A client SHOULD keep a list of hosts it cannot reach and
+   corresponding connection timeouts, rather than just retrying queued
+   mail items.
+
+   Experience suggests that failures are typically transient (the target
+   system or its connection has crashed), favoring a policy of two
+   connection attempts in the first hour the message is in the queue,
+   and then backing off to one every two or three hours.
+
+   The SMTP client can shorten the queuing delay in cooperation with the
+   SMTP server.  For example, if mail is received from a particular
+   address, it is likely that mail queued for that host can now be sent.
+   Application of this principle may, in many cases, eliminate the
+   requirement for an explicit "send queues now" function such as ETRN
+   [9].
+
+   The strategy may be further modified as a result of multiple
+   addresses per host (see below) to optimize delivery time vs. resource
+   usage.
+
+
+
+
+Klensin                     Standards Track                    [Page 58]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   An SMTP client may have a large queue of messages for each
+   unavailable destination host.  If all of these messages were retried
+   in every retry cycle, there would be excessive Internet overhead and
+   the sending system would be blocked for a long period.  Note that an
+   SMTP client can generally determine that a delivery attempt has
+   failed only after a timeout of several minutes and even a one-minute
+   timeout per connection will result in a very large delay if retries
+   are repeated for dozens, or even hundreds, of queued messages to the
+   same host.
+
+   At the same time, SMTP clients SHOULD use great care in caching
+   negative responses from servers.  In an extreme case, if EHLO is
+   issued multiple times during the same SMTP connection, different
+   answers may be returned by the server.  More significantly, 5yz
+   responses to the MAIL command MUST NOT be cached.
+
+   When a mail message is to be delivered to multiple recipients, and
+   the SMTP server to which a copy of the message is to be sent is the
+   same for multiple recipients, then only one copy of the message
+   SHOULD be transmitted.  That is, the SMTP client SHOULD use the
+   command sequence:  MAIL, RCPT, RCPT,... RCPT, DATA instead of the
+   sequence: MAIL, RCPT, DATA, ..., MAIL, RCPT, DATA.  However, if there
+   are very many addresses, a limit on the number of RCPT commands per
+   MAIL command MAY be imposed.  Implementation of this efficiency
+   feature is strongly encouraged.
+
+   Similarly, to achieve timely delivery, the SMTP client MAY support
+   multiple concurrent outgoing mail transactions.  However, some limit
+   may be appropriate to protect the host from devoting all its
+   resources to mail.
+
+4.5.4.2 Receiving Strategy
+
+   The SMTP server SHOULD attempt to keep a pending listen on the SMTP
+   port at all times.  This requires the support of multiple incoming
+   TCP connections for SMTP.  Some limit MAY be imposed but servers that
+   cannot handle more than one SMTP transaction at a time are not in
+   conformance with the intent of this specification.
+
+   As discussed above, when the SMTP server receives mail from a
+   particular host address, it could activate its own SMTP queuing
+   mechanisms to retry any mail pending for that host address.
+
+4.5.5   Messages with a null reverse-path
+
+   There are several types of notification messages which are required
+   by existing and proposed standards to be sent with a null reverse
+   path, namely non-delivery notifications as discussed in section 3.7,
+
+
+
+Klensin                     Standards Track                    [Page 59]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   other kinds of Delivery Status Notifications (DSNs) [24], and also
+   Message Disposition Notifications (MDNs) [10].  All of these kinds of
+   messages are notifications about a previous message, and they are
+   sent to the reverse-path of the previous mail message.  (If the
+   delivery of such a notification message fails, that usually indicates
+   a problem with the mail system of the host to which the notification
+   message is addressed.  For this reason, at some hosts the MTA is set
+   up to forward such failed notification messages to someone who is
+   able to fix problems with the mail system, e.g., via the postmaster
+   alias.)
+
+   All other types of messages (i.e., any message which is not required
+   by a standards-track RFC to have a null reverse-path) SHOULD be sent
+   with with a valid, non-null reverse-path.
+
+   Implementors of automated email processors should be careful to make
+   sure that the various kinds of messages with null reverse-path are
+   handled correctly, in particular such systems SHOULD NOT reply to
+   messages with null reverse-path.
+
+5. Address Resolution and Mail Handling
+
+   Once an SMTP client lexically identifies a domain to which mail will
+   be delivered for processing (as described in sections 3.6 and 3.7), a
+   DNS lookup MUST be performed to resolve the domain name [22].  The
+   names are expected to be fully-qualified domain names (FQDNs):
+   mechanisms for inferring FQDNs from partial names or local aliases
+   are outside of this specification and, due to a history of problems,
+   are generally discouraged.  The lookup first attempts to locate an MX
+   record associated with the name.  If a CNAME record is found instead,
+   the resulting name is processed as if it were the initial name.  If
+   no MX records are found, but an A RR is found, the A RR is treated as
+   if it was associated with an implicit MX RR, with a preference of 0,
+   pointing to that host.  If one or more MX RRs are found for a given
+   name, SMTP systems MUST NOT utilize any A RRs associated with that
+   name unless they are located using the MX RRs; the "implicit MX" rule
+   above applies only if there are no MX records present.  If MX records
+   are present, but none of them are usable, this situation MUST be
+   reported as an error.
+
+   When the lookup succeeds, the mapping can result in a list of
+   alternative delivery addresses rather than a single address, because
+   of multiple MX records, multihoming, or both.  To provide reliable
+   mail transmission, the SMTP client MUST be able to try (and retry)
+   each of the relevant addresses in this list in order, until a
+   delivery attempt succeeds.  However, there MAY also be a configurable
+   limit on the number of alternate addresses that can be tried.  In any
+   case, the SMTP client SHOULD try at least two addresses.
+
+
+
+Klensin                     Standards Track                    [Page 60]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   Two types of information is used to rank the host addresses: multiple
+   MX records, and multihomed hosts.
+
+   Multiple MX records contain a preference indication that MUST be used
+   in sorting (see below).  Lower numbers are more preferred than higher
+   ones.  If there are multiple destinations with the same preference
+   and there is no clear reason to favor one (e.g., by recognition of an
+   easily-reached address), then the sender-SMTP MUST randomize them to
+   spread the load across multiple mail exchangers for a specific
+   organization.
+
+   The destination host (perhaps taken from the preferred MX record) may
+   be multihomed, in which case the domain name resolver will return a
+   list of alternative IP addresses.  It is the responsibility of the
+   domain name resolver interface to have ordered this list by
+   decreasing preference if necessary, and SMTP MUST try them in the
+   order presented.
+
+   Although the capability to try multiple alternative addresses is
+   required, specific installations may want to limit or disable the use
+   of alternative addresses.  The question of whether a sender should
+   attempt retries using the different addresses of a multihomed host
+   has been controversial.  The main argument for using the multiple
+   addresses is that it maximizes the probability of timely delivery,
+   and indeed sometimes the probability of any delivery; the counter-
+   argument is that it may result in unnecessary resource use.  Note
+   that resource use is also strongly determined by the sending strategy
+   discussed in section 4.5.4.1.
+
+   If an SMTP server receives a message with a destination for which it
+   is a designated Mail eXchanger, it MAY relay the message (potentially
+   after having rewritten the MAIL FROM and/or RCPT TO addresses), make
+   final delivery of the message, or hand it off using some mechanism
+   outside the SMTP-provided transport environment.  Of course, neither
+   of the latter require that the list of MX records be examined
+   further.
+
+   If it determines that it should relay the message without rewriting
+   the address, it MUST sort the MX records to determine candidates for
+   delivery.  The records are first ordered by preference, with the
+   lowest-numbered records being most preferred.  The relay host MUST
+   then inspect the list for any of the names or addresses by which it
+   might be known in mail transactions.  If a matching record is found,
+   all records at that preference level and higher-numbered ones MUST be
+   discarded from consideration.  If there are no records left at that
+   point, it is an error condition, and the message MUST be returned as
+   undeliverable.  If records do remain, they SHOULD be tried, best
+   preference first, as described above.
+
+
+
+Klensin                     Standards Track                    [Page 61]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+6. Problem Detection and Handling
+
+6.1 Reliable Delivery and Replies by Email
+
+   When the receiver-SMTP accepts a piece of mail (by sending a "250 OK"
+   message in response to DATA), it is accepting responsibility for
+   delivering or relaying the message.  It must take this responsibility
+   seriously.  It MUST NOT lose the message for frivolous reasons, such
+   as because the host later crashes or because of a predictable
+   resource shortage.
+
+   If there is a delivery failure after acceptance of a message, the
+   receiver-SMTP MUST formulate and mail a notification message.  This
+   notification MUST be sent using a null ("<>") reverse path in the
+   envelope.  The recipient of this notification MUST be the address
+   from the envelope return path (or the Return-Path: line).  However,
+   if this address is null ("<>"), the receiver-SMTP MUST NOT send a
+   notification.  Obviously, nothing in this section can or should
+   prohibit local decisions (i.e., as part of the same system
+   environment as the receiver-SMTP) to log or otherwise transmit
+   information about null address events locally if that is desired.  If
+   the address is an explicit source route, it MUST be stripped down to
+   its final hop.
+
+   For example, suppose that an error notification must be sent for a
+   message that arrived with:
+
+      MAIL FROM:<@a,@b:user@d>
+
+   The notification message MUST be sent using:
+
+      RCPT TO:<user@d>
+
+   Some delivery failures after the message is accepted by SMTP will be
+   unavoidable.  For example, it may be impossible for the receiving
+   SMTP server to validate all the delivery addresses in RCPT command(s)
+   due to a "soft" domain system error, because the target is a mailing
+   list (see earlier discussion of RCPT), or because the server is
+   acting as a relay and has no immediate access to the delivering
+   system.
+
+   To avoid receiving duplicate messages as the result of timeouts, a
+   receiver-SMTP MUST seek to minimize the time required to respond to
+   the final <CRLF>.<CRLF> end of data indicator.  See RFC 1047 [28] for
+   a discussion of this problem.
+
+
+
+
+
+
+Klensin                     Standards Track                    [Page 62]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+6.2 Loop Detection
+
+   Simple counting of the number of "Received:" headers in a message has
+   proven to be an effective, although rarely optimal, method of
+   detecting loops in mail systems.  SMTP servers using this technique
+   SHOULD use a large rejection threshold, normally at least 100
+   Received entries.  Whatever mechanisms are used, servers MUST contain
+   provisions for detecting and stopping trivial loops.
+
+6.3 Compensating for Irregularities
+
+   Unfortunately, variations, creative interpretations, and outright
+   violations of Internet mail protocols do occur; some would suggest
+   that they occur quite frequently.  The debate as to whether a well-
+   behaved SMTP receiver or relay should reject a malformed message,
+   attempt to pass it on unchanged, or attempt to repair it to increase
+   the odds of successful delivery (or subsequent reply) began almost
+   with the dawn of structured network mail and shows no signs of
+   abating.  Advocates of rejection claim that attempted repairs are
+   rarely completely adequate and that rejection of bad messages is the
+   only way to get the offending software repaired.  Advocates of
+   "repair" or "deliver no matter what" argue that users prefer that
+   mail go through it if at all possible and that there are significant
+   market pressures in that direction.  In practice, these market
+   pressures may be more important to particular vendors than strict
+   conformance to the standards, regardless of the preference of the
+   actual developers.
+
+   The problems associated with ill-formed messages were exacerbated by
+   the introduction of the split-UA mail reading protocols [3, 26, 5,
+   21].  These protocols have encouraged the use of SMTP as a posting
+   protocol, and SMTP servers as relay systems for these client hosts
+   (which are often only intermittently connected to the Internet).
+   Historically, many of those client machines lacked some of the
+   mechanisms and information assumed by SMTP (and indeed, by the mail
+   format protocol [7]).  Some could not keep adequate track of time;
+   others had no concept of time zones; still others could not identify
+   their own names or addresses; and, of course, none could satisfy the
+   assumptions that underlay RFC 822's conception of authenticated
+   addresses.
+
+   In response to these weak SMTP clients, many SMTP systems now
+   complete messages that are delivered to them in incomplete or
+   incorrect form.  This strategy is generally considered appropriate
+   when the server can identify or authenticate the client, and there
+   are prior agreements between them.  By contrast, there is at best
+   great concern about fixes applied by a relay or delivery SMTP server
+   that has little or no knowledge of the user or client machine.
+
+
+
+Klensin                     Standards Track                    [Page 63]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   The following changes to a message being processed MAY be applied
+   when necessary by an originating SMTP server, or one used as the
+   target of SMTP as an initial posting protocol:
+
+   -  Addition of a message-id field when none appears
+
+   -  Addition of a date, time or time zone when none appears
+
+   -  Correction of addresses to proper FQDN format
+
+   The less information the server has about the client, the less likely
+   these changes are to be correct and the more caution and conservatism
+   should be applied when considering whether or not to perform fixes
+   and how.  These changes MUST NOT be applied by an SMTP server that
+   provides an intermediate relay function.
+
+   In all cases, properly-operating clients supplying correct
+   information are preferred to corrections by the SMTP server.  In all
+   cases, documentation of actions performed by the servers (in trace
+   fields and/or header comments) is strongly encouraged.
+
+7. Security Considerations
+
+7.1 Mail Security and Spoofing
+
+   SMTP mail is inherently insecure in that it is feasible for even
+   fairly casual users to negotiate directly with receiving and relaying
+   SMTP servers and create messages that will trick a naive recipient
+   into believing that they came from somewhere else.  Constructing such
+   a message so that the "spoofed" behavior cannot be detected by an
+   expert is somewhat more difficult, but not sufficiently so as to be a
+   deterrent to someone who is determined and knowledgeable.
+   Consequently, as knowledge of Internet mail increases, so does the
+   knowledge that SMTP mail inherently cannot be authenticated, or
+   integrity checks provided, at the transport level.  Real mail
+   security lies only in end-to-end methods involving the message
+   bodies, such as those which use digital signatures (see [14] and,
+   e.g., PGP [4] or S/MIME [31]).
+
+   Various protocol extensions and configuration options that provide
+   authentication at the transport level (e.g., from an SMTP client to
+   an SMTP server) improve somewhat on the traditional situation
+   described above.  However, unless they are accompanied by careful
+   handoffs of responsibility in a carefully-designed trust environment,
+   they remain inherently weaker than end-to-end mechanisms which use
+   digitally signed messages rather than depending on the integrity of
+   the transport system.
+
+
+
+
+Klensin                     Standards Track                    [Page 64]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   Efforts to make it more difficult for users to set envelope return
+   path and header "From" fields to point to valid addresses other than
+   their own are largely misguided: they frustrate legitimate
+   applications in which mail is sent by one user on behalf of another
+   or in which error (or normal) replies should be directed to a special
+   address.  (Systems that provide convenient ways for users to alter
+   these fields on a per-message basis should attempt to establish a
+   primary and permanent mailbox address for the user so that Sender
+   fields within the message data can be generated sensibly.)
+
+   This specification does not further address the authentication issues
+   associated with SMTP other than to advocate that useful functionality
+   not be disabled in the hope of providing some small margin of
+   protection against an ignorant user who is trying to fake mail.
+
+7.2 "Blind" Copies
+
+   Addresses that do not appear in the message headers may appear in the
+   RCPT commands to an SMTP server for a number of reasons.  The two
+   most common involve the use of a mailing address as a "list exploder"
+   (a single address that resolves into multiple addresses) and the
+   appearance of "blind copies".  Especially when more than one RCPT
+   command is present, and in order to avoid defeating some of the
+   purpose of these mechanisms, SMTP clients and servers SHOULD NOT copy
+   the full set of RCPT command arguments into the headers, either as
+   part of trace headers or as informational or private-extension
+   headers.  Since this rule is often violated in practice, and cannot
+   be enforced, sending SMTP systems that are aware of "bcc" use MAY
+   find it helpful to send each blind copy as a separate message
+   transaction containing only a single RCPT command.
+
+   There is no inherent relationship between either "reverse" (from
+   MAIL, SAML, etc., commands) or "forward" (RCPT) addresses in the SMTP
+   transaction ("envelope") and the addresses in the headers.  Receiving
+   systems SHOULD NOT attempt to deduce such relationships and use them
+   to alter the headers of the message for delivery.  The popular
+   "Apparently-to" header is a violation of this principle as well as a
+   common source of unintended information disclosure and SHOULD NOT be
+   used.
+
+7.3 VRFY, EXPN, and Security
+
+   As discussed in section 3.5, individual sites may want to disable
+   either or both of VRFY or EXPN for security reasons.  As a corollary
+   to the above, implementations that permit this MUST NOT appear to
+   have verified addresses that are not, in fact, verified.  If a site
+
+
+
+
+
+Klensin                     Standards Track                    [Page 65]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   disables these commands for security reasons, the SMTP server MUST
+   return a 252 response, rather than a code that could be confused with
+   successful or unsuccessful verification.
+
+   Returning a 250 reply code with the address listed in the VRFY
+   command after having checked it only for syntax violates this rule.
+   Of course, an implementation that "supports" VRFY by always returning
+   550 whether or not the address is valid is equally not in
+   conformance.
+
+   Within the last few years, the contents of mailing lists have become
+   popular as an address information source for so-called "spammers."
+   The use of EXPN to "harvest" addresses has increased as list
+   administrators have installed protections against inappropriate uses
+   of the lists themselves.  Implementations SHOULD still provide
+   support for EXPN, but sites SHOULD carefully evaluate the tradeoffs.
+   As authentication mechanisms are introduced into SMTP, some sites may
+   choose to make EXPN available only to authenticated requestors.
+
+7.4 Information Disclosure in Announcements
+
+   There has been an ongoing debate about the tradeoffs between the
+   debugging advantages of announcing server type and version (and,
+   sometimes, even server domain name) in the greeting response or in
+   response to the HELP command and the disadvantages of exposing
+   information that might be useful in a potential hostile attack.  The
+   utility of the debugging information is beyond doubt.  Those who
+   argue for making it available point out that it is far better to
+   actually secure an SMTP server rather than hope that trying to
+   conceal known vulnerabilities by hiding the server's precise identity
+   will provide more protection.  Sites are encouraged to evaluate the
+   tradeoff with that issue in mind; implementations are strongly
+   encouraged to minimally provide for making type and version
+   information available in some way to other network hosts.
+
+7.5 Information Disclosure in Trace Fields
+
+   In some circumstances, such as when mail originates from within a LAN
+   whose hosts are not directly on the public Internet, trace
+   ("Received") fields produced in conformance with this specification
+   may disclose host names and similar information that would not
+   normally be available.  This ordinarily does not pose a problem, but
+   sites with special concerns about name disclosure should be aware of
+   it.  Also, the optional FOR clause should be supplied with caution or
+   not at all when multiple recipients are involved lest it
+   inadvertently disclose the identities of "blind copy" recipients to
+   others.
+
+
+
+
+Klensin                     Standards Track                    [Page 66]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+7.6 Information Disclosure in Message Forwarding
+
+   As discussed in section 3.4, use of the 251 or 551 reply codes to
+   identify the replacement address associated with a mailbox may
+   inadvertently disclose sensitive information.  Sites that are
+   concerned about those issues should ensure that they select and
+   configure servers appropriately.
+
+7.7 Scope of Operation of SMTP Servers
+
+   It is a well-established principle that an SMTP server may refuse to
+   accept mail for any operational or technical reason that makes sense
+   to the site providing the server.  However, cooperation among sites
+   and installations makes the Internet possible.  If sites take
+   excessive advantage of the right to reject traffic, the ubiquity of
+   email availability (one of the strengths of the Internet) will be
+   threatened; considerable care should be taken and balance maintained
+   if a site decides to be selective about the traffic it will accept
+   and process.
+
+   In recent years, use of the relay function through arbitrary sites
+   has been used as part of hostile efforts to hide the actual origins
+   of mail.  Some sites have decided to limit the use of the relay
+   function to known or identifiable sources, and implementations SHOULD
+   provide the capability to perform this type of filtering.  When mail
+   is rejected for these or other policy reasons, a 550 code SHOULD be
+   used in response to EHLO, MAIL, or RCPT as appropriate.
+
+8. IANA Considerations
+
+   IANA will maintain three registries in support of this specification.
+   The first consists of SMTP service extensions with the associated
+   keywords, and, as needed, parameters and verbs.  As specified in
+   section 2.2.2, no entry may be made in this registry that starts in
+   an "X".  Entries may be made only for service extensions (and
+   associated keywords, parameters, or verbs) that are defined in
+   standards-track or experimental RFCs specifically approved by the
+   IESG for this purpose.
+
+   The second registry consists of "tags" that identify forms of domain
+   literals other than those for IPv4 addresses (specified in RFC 821
+   and in this document) and IPv6 addresses (specified in this
+   document).  Additional literal types require standardization before
+   being used; none are anticipated at this time.
+
+   The third, established by RFC 821 and renewed by this specification,
+   is a registry of link and protocol identifiers to be used with the
+   "via" and "with" subclauses of the time stamp ("Received: header")
+
+
+
+Klensin                     Standards Track                    [Page 67]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   described in section 4.4.  Link and protocol identifiers in addition
+   to those specified in this document may be registered only by
+   standardization or by way of an RFC-documented, IESG-approved,
+   Experimental protocol extension.
+
+9. References
+
+   [1]  American National Standards Institute (formerly United States of
+        America Standards Institute), X3.4, 1968, "USA Code for
+        Information Interchange". ANSI X3.4-1968 has been replaced by
+        newer versions with slight modifications, but the 1968 version
+        remains definitive for the Internet.
+
+   [2]  Braden, R., "Requirements for Internet hosts - application and
+        support", STD 3, RFC 1123, October 1989.
+
+   [3]  Butler, M., Chase, D., Goldberger, J., Postel, J. and J.
+        Reynolds, "Post Office Protocol - version 2", RFC 937, February
+        1985.
+
+   [4]  Callas, J., Donnerhacke, L., Finney, H. and R. Thayer, "OpenPGP
+        Message Format", RFC 2440, November 1998.
+
+   [5]  Crispin, M., "Interactive Mail Access Protocol - Version 2", RFC
+        1176, August 1990.
+
+   [6]  Crispin, M., "Internet Message Access Protocol - Version 4", RFC
+        2060, December 1996.
+
+   [7]  Crocker, D., "Standard for the Format of ARPA Internet Text
+        Messages", RFC 822, August 1982.
+
+   [8]  Crocker, D. and P. Overell, Eds., "Augmented BNF for Syntax
+        Specifications: ABNF", RFC 2234, November 1997.
+
+   [9]  De Winter, J., "SMTP Service Extension for Remote Message Queue
+        Starting", RFC 1985, August 1996.
+
+   [10] Fajman, R., "An Extensible Message Format for Message
+        Disposition Notifications", RFC 2298, March 1998.
+
+   [11] Freed, N, "Behavior of and Requirements for Internet Firewalls",
+        RFC 2979, October 2000.
+
+   [12] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
+        Extensions (MIME) Part One: Format of Internet Message Bodies",
+        RFC 2045, December 1996.
+
+
+
+
+Klensin                     Standards Track                    [Page 68]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   [13] Freed, N., "SMTP Service Extension for Command Pipelining", RFC
+        2920, September 2000.
+
+   [14] Galvin, J., Murphy, S., Crocker, S. and N. Freed, "Security
+        Multiparts for MIME: Multipart/Signed and Multipart/Encrypted",
+        RFC 1847, October 1995.
+
+   [15] Gellens, R. and J. Klensin, "Message Submission", RFC 2476,
+        December 1998.
+
+   [16] Kille, S., "Mapping between X.400 and RFC822/MIME", RFC 2156,
+        January 1998.
+
+   [17] Hinden, R and S. Deering, Eds. "IP Version 6 Addressing
+        Architecture", RFC 2373, July 1998.
+
+   [18] Klensin, J., Freed, N. and K. Moore, "SMTP Service Extension for
+        Message Size Declaration", STD 10, RFC 1870, November 1995.
+
+   [19] Klensin, J., Freed, N., Rose, M., Stefferud, E. and D. Crocker,
+        "SMTP Service Extensions", STD 10, RFC 1869, November 1995.
+
+   [20] Klensin, J., Freed, N., Rose, M., Stefferud, E. and D. Crocker,
+        "SMTP Service Extension for 8bit-MIMEtransport", RFC 1652, July
+        1994.
+
+   [21] Lambert, M., "PCMAIL: A distributed mail system for personal
+        computers", RFC 1056, July 1988.
+
+   [22] Mockapetris, P., "Domain names - implementation and
+        specification", STD 13, RFC 1035, November 1987.
+
+        Mockapetris, P., "Domain names - concepts and facilities", STD
+        13, RFC 1034, November 1987.
+
+   [23] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part
+        Three: Message Header Extensions for Non-ASCII Text", RFC 2047,
+        December 1996.
+
+   [24] Moore, K., "SMTP Service Extension for Delivery Status
+        Notifications", RFC 1891, January 1996.
+
+   [25] Moore, K., and G. Vaudreuil, "An Extensible Message Format for
+        Delivery Status Notifications", RFC 1894, January 1996.
+
+   [26] Myers, J. and M. Rose, "Post Office Protocol - Version 3", STD
+        53, RFC 1939, May 1996.
+
+
+
+
+Klensin                     Standards Track                    [Page 69]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   [27] Partridge, C., "Mail routing and the domain system", RFC 974,
+        January 1986.
+
+   [28] Partridge, C., "Duplicate messages and SMTP", RFC 1047, February
+        1988.
+
+   [29] Postel, J., ed., "Transmission Control Protocol - DARPA Internet
+        Program Protocol Specification", STD 7, RFC 793, September 1981.
+
+   [30] Postel, J., "Simple Mail Transfer Protocol", RFC 821, August
+        1982.
+
+   [31] Ramsdell, B., Ed., "S/MIME Version 3 Message Specification", RFC
+        2633, June 1999.
+
+   [32] Resnick, P., Ed., "Internet Message Format", RFC 2822, April
+        2001.
+
+   [33] Vaudreuil, G., "SMTP Service Extensions for Transmission of
+        Large and Binary MIME Messages", RFC 1830, August 1995.
+
+   [34] Vaudreuil, G., "Enhanced Mail System Status Codes", RFC 1893,
+        January 1996.
+
+10. Editor's Address
+
+   John C. Klensin
+   AT&T Laboratories
+   99 Bedford St
+   Boston, MA 02111 USA
+
+   Phone: 617-574-3076
+   EMail: klensin@research.att.com
+
+11. Acknowledgments
+
+   Many people worked long and hard on the many iterations of this
+   document.  There was wide-ranging debate in the IETF DRUMS Working
+   Group, both on its mailing list and in face to face discussions,
+   about many technical issues and the role of a revised standard for
+   Internet mail transport, and many contributors helped form the
+   wording in this specification.  The hundreds of participants in the
+   many discussions since RFC 821 was produced are too numerous to
+   mention, but they all helped this document become what it is.
+
+
+
+
+
+
+
+Klensin                     Standards Track                    [Page 70]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+APPENDICES
+
+A. TCP Transport Service
+
+   The TCP connection supports the transmission of 8-bit bytes.  The
+   SMTP data is 7-bit ASCII characters.  Each character is transmitted
+   as an 8-bit byte with the high-order bit cleared to zero.  Service
+   extensions may modify this rule to permit transmission of full 8-bit
+   data bytes as part of the message body, but not in SMTP commands or
+   responses.
+
+B. Generating SMTP Commands from RFC 822 Headers
+
+   Some systems use RFC 822 headers (only) in a mail submission
+   protocol, or otherwise generate SMTP commands from RFC 822 headers
+   when such a message is handed to an MTA from a UA.  While the MTA-UA
+   protocol is a private matter, not covered by any Internet Standard,
+   there are problems with this approach.  For example, there have been
+   repeated problems with proper handling of "bcc" copies and
+   redistribution lists when information that conceptually belongs to a
+   mail envelopes is not separated early in processing from header
+   information (and kept separate).
+
+   It is recommended that the UA provide its initial ("submission
+   client") MTA with an envelope separate from the message itself.
+   However, if the envelope is not supplied, SMTP commands SHOULD be
+   generated as follows:
+
+   1. Each recipient address from a TO, CC, or BCC header field SHOULD
+      be copied to a RCPT command (generating multiple message copies if
+      that is required for queuing or delivery).  This includes any
+      addresses listed in a RFC 822 "group".  Any BCC fields SHOULD then
+      be removed from the headers.  Once this process is completed, the
+      remaining headers SHOULD be checked to verify that at least one
+      To:, Cc:, or Bcc: header remains.  If none do, then a bcc: header
+      with no additional information SHOULD be inserted as specified in
+      [32].
+
+   2. The return address in the MAIL command SHOULD, if possible, be
+      derived from the system's identity for the submitting (local)
+      user, and the "From:" header field otherwise.  If there is a
+      system identity available, it SHOULD also be copied to the Sender
+      header field if it is different from the address in the From
+      header field.  (Any Sender field that was already there SHOULD be
+      removed.)  Systems may provide a way for submitters to override
+      the envelope return address, but may want to restrict its use to
+      privileged users.  This will not prevent mail forgery, but may
+      lessen its incidence; see section 7.1.
+
+
+
+Klensin                     Standards Track                    [Page 71]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   When an MTA is being used in this way, it bears responsibility for
+   ensuring that the message being transmitted is valid.  The mechanisms
+   for checking that validity, and for handling (or returning) messages
+   that are not valid at the time of arrival, are part of the MUA-MTA
+   interface and not covered by this specification.
+
+   A submission protocol based on Standard RFC 822 information alone
+   MUST NOT be used to gateway a message from a foreign (non-SMTP) mail
+   system into an SMTP environment.  Additional information to construct
+   an envelope must come from some source in the other environment,
+   whether supplemental headers or the foreign system's envelope.
+
+   Attempts to gateway messages using only their header "to" and "cc"
+   fields have repeatedly caused mail loops and other behavior adverse
+   to the proper functioning of the Internet mail environment.  These
+   problems have been especially common when the message originates from
+   an Internet mailing list and is distributed into the foreign
+   environment using envelope information.  When these messages are then
+   processed by a header-only remailer, loops back to the Internet
+   environment (and the mailing list) are almost inevitable.
+
+C. Source Routes
+
+   Historically, the <reverse-path> was a reverse source routing list of
+   hosts and a source mailbox.  The first host in the <reverse-path>
+   SHOULD be the host sending the MAIL command.  Similarly, the
+   <forward-path> may be a source routing lists of hosts and a
+   destination mailbox.  However, in general, the <forward-path> SHOULD
+   contain only a mailbox and domain name, relying on the domain name
+   system to supply routing information if required.  The use of source
+   routes is deprecated; while servers MUST be prepared to receive and
+   handle them as discussed in section 3.3 and F.2, clients SHOULD NOT
+   transmit them and this section was included only to provide context.
+
+   For relay purposes, the forward-path may be a source route of the
+   form "@ONE,@TWO:JOE@THREE", where ONE, TWO, and THREE MUST BE fully-
+   qualified domain names.  This form is used to emphasize the
+   distinction between an address and a route.  The mailbox is an
+   absolute address, and the route is information about how to get
+   there.  The two concepts should not be confused.
+
+   If source routes are used, RFC 821 and the text below should be
+   consulted for the mechanisms for constructing and updating the
+   forward- and reverse-paths.
+
+
+
+
+
+
+
+Klensin                     Standards Track                    [Page 72]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+   The SMTP server transforms the command arguments by moving its own
+   identifier (its domain name or that of any domain for which it is
+   acting as a mail exchanger), if it appears, from the forward-path to
+   the beginning of the reverse-path.
+
+   Notice that the forward-path and reverse-path appear in the SMTP
+   commands and replies, but not necessarily in the message.  That is,
+   there is no need for these paths and especially this syntax to appear
+   in the "To:" , "From:", "CC:", etc. fields of the message header.
+   Conversely, SMTP servers MUST NOT derive final message delivery
+   information from message header fields.
+
+   When the list of hosts is present, it is a "reverse" source route and
+   indicates that the mail was relayed through each host on the list
+   (the first host in the list was the most recent relay).  This list is
+   used as a source route to return non-delivery notices to the sender.
+   As each relay host adds itself to the beginning of the list, it MUST
+   use its name as known in the transport environment to which it is
+   relaying the mail rather than that of the transport environment from
+   which the mail came (if they are different).
+
+D. Scenarios
+
+   This section presents complete scenarios of several types of SMTP
+   sessions.  In the examples, "C:" indicates what is said by the SMTP
+   client, and "S:" indicates what is said by the SMTP server.
+
+D.1 A Typical SMTP Transaction Scenario
+
+   This SMTP example shows mail sent by Smith at host bar.com, to Jones,
+   Green, and Brown at host foo.com.  Here we assume that host bar.com
+   contacts host foo.com directly.  The mail is accepted for Jones and
+   Brown.  Green does not have a mailbox at host foo.com.
+
+      S: 220 foo.com Simple Mail Transfer Service Ready
+      C: EHLO bar.com
+      S: 250-foo.com greets bar.com
+      S: 250-8BITMIME
+      S: 250-SIZE
+      S: 250-DSN
+      S: 250 HELP
+      C: MAIL FROM:<Smith@bar.com>
+      S: 250 OK
+      C: RCPT TO:<Jones@foo.com>
+      S: 250 OK
+      C: RCPT TO:<Green@foo.com>
+      S: 550 No such user here
+      C: RCPT TO:<Brown@foo.com>
+
+
+
+Klensin                     Standards Track                    [Page 73]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+      S: 250 OK
+      C: DATA
+      S: 354 Start mail input; end with <CRLF>.<CRLF>
+      C: Blah blah blah...
+      C: ...etc. etc. etc.
+      C: .
+      S: 250 OK
+      C: QUIT
+      S: 221 foo.com Service closing transmission channel
+
+D.2 Aborted SMTP Transaction Scenario
+
+      S: 220 foo.com Simple Mail Transfer Service Ready
+      C: EHLO bar.com
+      S: 250-foo.com greets bar.com
+      S: 250-8BITMIME
+      S: 250-SIZE
+      S: 250-DSN
+      S: 250 HELP
+      C: MAIL FROM:<Smith@bar.com>
+      S: 250 OK
+      C: RCPT TO:<Jones@foo.com>
+      S: 250 OK
+      C: RCPT TO:<Green@foo.com>
+      S: 550 No such user here
+      C: RSET
+      S: 250 OK
+      C: QUIT
+      S: 221 foo.com Service closing transmission channel
+
+D.3 Relayed Mail Scenario
+
+   Step 1  --  Source Host to Relay Host
+
+      S: 220 foo.com Simple Mail Transfer Service Ready
+      C: EHLO bar.com
+      S: 250-foo.com greets bar.com
+      S: 250-8BITMIME
+      S: 250-SIZE
+      S: 250-DSN
+      S: 250 HELP
+      C: MAIL FROM:<JQP@bar.com>
+      S: 250 OK
+      C: RCPT TO:<@foo.com:Jones@XYZ.COM>
+      S: 250 OK
+      C: DATA
+      S: 354 Start mail input; end with <CRLF>.<CRLF>
+      C: Date: Thu, 21 May 1998 05:33:29 -0700
+
+
+
+Klensin                     Standards Track                    [Page 74]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+      C: From: John Q. Public <JQP@bar.com>
+      C: Subject:  The Next Meeting of the Board
+      C: To: Jones@xyz.com
+      C:
+      C: Bill:
+      C: The next meeting of the board of directors will be
+      C: on Tuesday.
+      C:                         John.
+      C: .
+      S: 250 OK
+      C: QUIT
+      S: 221 foo.com Service closing transmission channel
+
+   Step 2  --  Relay Host to Destination Host
+
+      S: 220 xyz.com Simple Mail Transfer Service Ready
+      C: EHLO foo.com
+      S: 250 xyz.com is on the air
+      C: MAIL FROM:<@foo.com:JQP@bar.com>
+      S: 250 OK
+      C: RCPT TO:<Jones@XYZ.COM>
+      S: 250 OK
+      C: DATA
+      S: 354 Start mail input; end with <CRLF>.<CRLF>
+      C: Received: from bar.com by foo.com ; Thu, 21 May 1998
+      C:     05:33:29 -0700
+      C: Date: Thu, 21 May 1998 05:33:22 -0700
+      C: From: John Q. Public <JQP@bar.com>
+      C: Subject:  The Next Meeting of the Board
+      C: To: Jones@xyz.com
+      C:
+      C: Bill:
+      C: The next meeting of the board of directors will be
+      C: on Tuesday.
+      C:                         John.
+      C: .
+      S: 250 OK
+      C: QUIT
+      S: 221 foo.com Service closing transmission channel
+
+D.4 Verifying and Sending Scenario
+
+      S: 220 foo.com Simple Mail Transfer Service Ready
+      C: EHLO bar.com
+      S: 250-foo.com greets bar.com
+      S: 250-8BITMIME
+      S: 250-SIZE
+      S: 250-DSN
+
+
+
+Klensin                     Standards Track                    [Page 75]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+      S: 250-VRFY
+      S: 250 HELP
+      C: VRFY Crispin
+      S: 250 Mark Crispin <Admin.MRC@foo.com>
+      C: SEND FROM:<EAK@bar.com>
+      S: 250 OK
+      C: RCPT TO:<Admin.MRC@foo.com>
+      S: 250 OK
+      C: DATA
+      S: 354 Start mail input; end with <CRLF>.<CRLF>
+      C: Blah blah blah...
+      C: ...etc. etc. etc.
+      C: .
+      S: 250 OK
+      C: QUIT
+      S: 221 foo.com Service closing transmission channel
+
+E. Other Gateway Issues
+
+   In general, gateways between the Internet and other mail systems
+   SHOULD attempt to preserve any layering semantics across the
+   boundaries between the two mail systems involved.  Gateway-
+   translation approaches that attempt to take shortcuts by mapping,
+   (such as envelope information from one system to the message headers
+   or body of another) have generally proven to be inadequate in
+   important ways.  Systems translating between environments that do not
+   support both envelopes and headers and Internet mail must be written
+   with the understanding that some information loss is almost
+   inevitable.
+
+F. Deprecated Features of RFC 821
+
+   A few features of RFC 821 have proven to be problematic and SHOULD
+   NOT be used in Internet mail.
+
+F.1 TURN
+
+   This command, described in RFC 821, raises important security issues
+   since, in the absence of strong authentication of the host requesting
+   that the client and server switch roles, it can easily be used to
+   divert mail from its correct destination.  Its use is deprecated;
+   SMTP systems SHOULD NOT use it unless the server can authenticate the
+   client.
+
+
+
+
+
+
+
+
+Klensin                     Standards Track                    [Page 76]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+F.2 Source Routing
+
+   RFC 821 utilized the concept of explicit source routing to get mail
+   from one host to another via a series of relays.  The requirement to
+   utilize source routes in regular mail traffic was eliminated by the
+   introduction of the domain name system "MX" record and the last
+   significant justification for them was eliminated by the
+   introduction, in RFC 1123, of a clear requirement that addresses
+   following an "@" must all be fully-qualified domain names.
+   Consequently, the only remaining justifications for the use of source
+   routes are support for very old SMTP clients or MUAs and in mail
+   system debugging.  They can, however, still be useful in the latter
+   circumstance and for routing mail around serious, but temporary,
+   problems such as problems with the relevant DNS records.
+
+   SMTP servers MUST continue to accept source route syntax as specified
+   in the main body of this document and in RFC 1123.  They MAY, if
+   necessary, ignore the routes and utilize only the target domain in
+   the address.  If they do utilize the source route, the message MUST
+   be sent to the first domain shown in the address.  In particular, a
+   server MUST NOT guess at shortcuts within the source route.
+
+   Clients SHOULD NOT utilize explicit source routing except under
+   unusual circumstances, such as debugging or potentially relaying
+   around firewall or mail system configuration errors.
+
+F.3 HELO
+
+   As discussed in sections 3.1 and 4.1.1, EHLO is strongly preferred to
+   HELO when the server will accept the former.  Servers must continue
+   to accept and process HELO in order to support older clients.
+
+F.4 #-literals
+
+   RFC 821 provided for specifying an Internet address as a decimal
+   integer host number prefixed by a pound sign, "#".  In practice, that
+   form has been obsolete since the introduction of TCP/IP.  It is
+   deprecated and MUST NOT be used.
+
+F.5 Dates and Years
+
+   When dates are inserted into messages by SMTP clients or servers
+   (e.g., in trace fields), four-digit years MUST BE used.  Two-digit
+   years are deprecated; three-digit years were never permitted in the
+   Internet mail system.
+
+
+
+
+
+
+Klensin                     Standards Track                    [Page 77]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+F.6 Sending versus Mailing
+
+   In addition to specifying a mechanism for delivering messages to
+   user's mailboxes, RFC 821 provided additional, optional, commands to
+   deliver messages directly to the user's terminal screen.  These
+   commands (SEND, SAML, SOML) were rarely implemented, and changes in
+   workstation technology and the introduction of other protocols may
+   have rendered them obsolete even where they are implemented.
+
+   Clients SHOULD NOT provide SEND, SAML, or SOML as services.  Servers
+   MAY implement them.  If they are implemented by servers, the
+   implementation model specified in RFC 821 MUST be used and the
+   command names MUST be published in the response to the EHLO command.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Klensin                     Standards Track                    [Page 78]
+
+RFC 2821             Simple Mail Transfer Protocol            April 2001
+
+
+Full Copyright Statement
+
+   Copyright (C) The Internet Society (2001).  All Rights Reserved.
+
+   This document and translations of it may be copied and furnished to
+   others, and derivative works that comment on or otherwise explain it
+   or assist in its implementation may be prepared, copied, published
+   and distributed, in whole or in part, without restriction of any
+   kind, provided that the above copyright notice and this paragraph are
+   included on all such copies and derivative works.  However, this
+   document itself may not be modified in any way, such as by removing
+   the copyright notice or references to the Internet Society or other
+   Internet organizations, except as needed for the purpose of
+   developing Internet standards in which case the procedures for
+   copyrights defined in the Internet Standards process must be
+   followed, or as required to translate it into languages other than
+   English.
+
+   The limited permissions granted above are perpetual and will not be
+   revoked by the Internet Society or its successors or assigns.
+
+   This document and the information contained herein is provided on an
+   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
+   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
+   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
+   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
+   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
+
+Acknowledgement
+
+   Funding for the RFC Editor function is currently provided by the
+   Internet Society.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Klensin                     Standards Track                    [Page 79]
+