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diff --git a/doc/rfc/rfc2821.txt b/doc/rfc/rfc2821.txt new file mode 100644 index 0000000..0eac911 --- /dev/null +++ b/doc/rfc/rfc2821.txt @@ -0,0 +1,4427 @@ + + + + + + +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] + |