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+
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+ RFC 821
+
+
+
+
+
+ SIMPLE MAIL TRANSFER PROTOCOL
+
+
+
+ Jonathan B. Postel
+
+
+
+
+
+
+
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+
+
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+
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+ August 1982
+
+
+
+ Information Sciences Institute
+ University of Southern California
+ 4676 Admiralty Way
+ Marina del Rey, California 90291
+
+ (213) 822-1511
+
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+ TABLE OF CONTENTS
+
+ 1. INTRODUCTION .................................................. 1
+
+ 2. THE SMTP MODEL ................................................ 2
+
+ 3. THE SMTP PROCEDURE ............................................ 4
+
+ 3.1. Mail ..................................................... 4
+ 3.2. Forwarding ............................................... 7
+ 3.3. Verifying and Expanding .................................. 8
+ 3.4. Sending and Mailing ..................................... 11
+ 3.5. Opening and Closing ..................................... 13
+ 3.6. Relaying ................................................ 14
+ 3.7. Domains ................................................. 17
+ 3.8. Changing Roles .......................................... 18
+
+ 4. THE SMTP SPECIFICATIONS ...................................... 19
+
+ 4.1. SMTP Commands ........................................... 19
+ 4.1.1. Command Semantics ..................................... 19
+ 4.1.2. Command Syntax ........................................ 27
+ 4.2. SMTP Replies ............................................ 34
+ 4.2.1. Reply Codes by Function Group ......................... 35
+ 4.2.2. Reply Codes in Numeric Order .......................... 36
+ 4.3. Sequencing of Commands and Replies ...................... 37
+ 4.4. State Diagrams .......................................... 39
+ 4.5. Details ................................................. 41
+ 4.5.1. Minimum Implementation ................................ 41
+ 4.5.2. Transparency .......................................... 41
+ 4.5.3. Sizes ................................................. 42
+
+ APPENDIX A: TCP ................................................. 44
+ APPENDIX B: NCP ................................................. 45
+ APPENDIX C: NITS ................................................ 46
+ APPENDIX D: X.25 ................................................ 47
+ APPENDIX E: Theory of Reply Codes ............................... 48
+ APPENDIX F: Scenarios ........................................... 51
+
+ GLOSSARY ......................................................... 64
+
+ REFERENCES ....................................................... 67
+
+
+
+
+Network Working Group J. Postel
+Request for Comments: DRAFT ISI
+Replaces: RFC 788, 780, 772 August 1982
+
+ SIMPLE MAIL TRANSFER PROTOCOL
+
+
+1. INTRODUCTION
+
+ The objective of 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. Appendices A,
+ B, C, and D describe the use of SMTP with various transport services.
+ A Glossary provides the definitions of terms as used in this
+ document.
+
+ An important feature of SMTP is its capability to relay mail across
+ transport service environments. A transport service provides an
+ interprocess communication environment (IPCE). An IPCE may cover one
+ network, several networks, or a subset of a network. It is important
+ to realize that transport systems (or IPCEs) are not one-to-one with
+ networks. A process can communicate directly with another process
+ through any mutually known IPCE. Mail is an application or use of
+ interprocess communication. Mail can be communicated between
+ processes in different IPCEs by relaying through a process connected
+ to two (or more) IPCEs. More specifically, mail can be relayed
+ between hosts on different transport systems by a host on both
+ transport systems.
+
+
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+Postel [Page 1]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+2. THE SMTP MODEL
+
+ The SMTP design is based on the following model of communication: as
+ the result of a user mail request, the sender-SMTP establishes a
+ two-way transmission channel to a receiver-SMTP. The receiver-SMTP
+ may be either the ultimate destination or an intermediate. SMTP
+ commands are generated by the sender-SMTP and sent to the
+ receiver-SMTP. SMTP replies are sent from the receiver-SMTP to the
+ sender-SMTP in response to the commands.
+
+ Once the transmission channel is established, the SMTP-sender sends a
+ MAIL command indicating the sender of the mail. If the SMTP-receiver
+ can accept mail it responds with an OK reply. The SMTP-sender then
+ sends a RCPT command identifying a recipient of the mail. If the
+ SMTP-receiver can accept mail for that recipient it responds with an
+ OK reply; if not, it responds with a reply rejecting that recipient
+ (but not the whole mail transaction). The SMTP-sender and
+ SMTP-receiver may negotiate several recipients. When the recipients
+ have been negotiated the SMTP-sender sends the mail data, terminating
+ with a special sequence. If the SMTP-receiver successfully processes
+ the mail data it responds with an OK reply. The dialog is purposely
+ lock-step, one-at-a-time.
+
+ -------------------------------------------------------------
+
+
+ +----------+ +----------+
+ +------+ | | | |
+ | User |<-->| | SMTP | |
+ +------+ | Sender- |Commands/Replies| Receiver-|
+ +------+ | SMTP |<-------------->| SMTP | +------+
+ | File |<-->| | and Mail | |<-->| File |
+ |System| | | | | |System|
+ +------+ +----------+ +----------+ +------+
+
+
+ Sender-SMTP Receiver-SMTP
+
+ Model for SMTP Use
+
+ Figure 1
+
+ -------------------------------------------------------------
+
+ The SMTP provides mechanisms for the transmission of mail; directly
+ from the sending user's host to the receiving user's host when the
+
+
+
+[Page 2] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+ two host are connected to the same transport service, or via one or
+ more relay SMTP-servers when the source and destination hosts are not
+ connected to the same transport service.
+
+ To be able to provide the relay capability the SMTP-server must be
+ supplied with the name of the ultimate destination host as well as
+ the destination mailbox name.
+
+ The argument to the MAIL command is a reverse-path, which specifies
+ who the mail is from. The argument to the RCPT command is a
+ forward-path, which specifies who the mail is to. The forward-path
+ is a source route, while the reverse-path is a return route (which
+ may be used to return a message to the sender when an error occurs
+ with a relayed message).
+
+ When the same message is sent to multiple recipients the SMTP
+ encourages the transmission of only one copy of the data for all the
+ recipients at the same destination host.
+
+ The mail commands and replies have a rigid syntax. Replies also have
+ a numeric code. In the following, examples appear which use actual
+ commands and replies. The complete lists of commands and replies
+ appears in Section 4 on specifications.
+
+ Commands and replies are not case sensitive. That is, a command or
+ reply word may be upper case, lower case, or any mixture of upper and
+ lower case. Note that this is not true of mailbox user names. For
+ some hosts the user name is case sensitive, and SMTP implementations
+ must take case to preserve the case of user names as they appear in
+ mailbox arguments. Host names are not case sensitive.
+
+ 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.
+
+ When specifying the general form of a command or reply, an argument
+ (or special symbol) will be denoted by a meta-linguistic variable (or
+ constant), for example, "<string>" or "<reverse-path>". Here the
+ angle brackets indicate these are meta-linguistic variables.
+ However, some arguments use the angle brackets literally. For
+ example, an actual reverse-path is enclosed in angle brackets, i.e.,
+ "<John.Smith@USC-ISI.ARPA>" is an instance of <reverse-path> (the
+ angle brackets are actually transmitted in the command or reply).
+
+
+
+
+
+Postel [Page 3]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+3. THE SMTP PROCEDURES
+
+ This section presents the procedures used in SMTP in several parts.
+ First comes the basic mail procedure defined as a mail transaction.
+ Following this are descriptions of forwarding mail, verifying mailbox
+ names and expanding mailing lists, sending to terminals instead of or
+ in combination with mailboxes, and the opening and closing exchanges.
+ At the end of this section are comments on relaying, a note on mail
+ domains, and a discussion of changing roles. Throughout this section
+ are examples of partial command and reply sequences, several complete
+ scenarios are presented in Appendix F.
+
+ 3.1. MAIL
+
+ There are three steps to SMTP mail transactions. The transaction
+ is started with a MAIL command which gives the sender
+ identification. A series of one or more RCPT commands follows
+ giving the receiver information. Then a DATA command gives the
+ mail data. And finally, the end of mail data indicator confirms
+ the transaction.
+
+ The first step in the procedure is the MAIL command. The
+ <reverse-path> contains the source mailbox.
+
+ MAIL <SP> FROM:<reverse-path> <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. It gives the
+ reverse-path which can be used to report errors. If accepted,
+ the receiver-SMTP returns a 250 OK reply.
+
+ The <reverse-path> can contain more than just a mailbox. The
+ <reverse-path> is a reverse source routing list of hosts and
+ source mailbox. The first host in the <reverse-path> should be
+ the host sending this command.
+
+ The second step in the procedure is the RCPT command.
+
+ RCPT <SP> TO:<forward-path> <CRLF>
+
+ This command gives a forward-path identifying one recipient.
+ If accepted, the receiver-SMTP returns a 250 OK reply, and
+ stores the forward-path. If the recipient is unknown the
+ receiver-SMTP returns a 550 Failure reply. This second step of
+ the procedure can be repeated any number of times.
+
+
+
+[Page 4] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+ The <forward-path> can contain more than just a mailbox. The
+ <forward-path> is a source routing list of hosts and the
+ destination mailbox. The first host in the <forward-path>
+ should be the host receiving this command.
+
+ The third step in the procedure is the DATA command.
+
+ DATA <CRLF>
+
+ If accepted, the receiver-SMTP returns a 354 Intermediate reply
+ and considers all succeeding lines to be the message text.
+ When the end of text is received and stored the SMTP-receiver
+ sends a 250 OK reply.
+
+ Since the mail data is sent on the transmission channel the end
+ of the 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. A
+ transparency procedure is used to prevent this from interfering
+ with the user's text (see Section 4.5.2).
+
+ Please note that the mail data includes the memo header
+ items such as Date, Subject, To, Cc, From [2].
+
+ The end of mail data indicator also confirms the mail
+ transaction and tells the receiver-SMTP to now process the
+ stored recipients and mail data. If accepted, the
+ receiver-SMTP returns a 250 OK reply. The DATA command should
+ fail only if the mail transaction was incomplete (for example,
+ no recipients), or if resources are not available.
+
+ The above procedure is an example of a mail transaction. These
+ commands must be used only in the order discussed above.
+ Example 1 (below) illustrates the use of these commands in a mail
+ transaction.
+
+
+
+
+
+
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+
+Postel [Page 5]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+ -------------------------------------------------------------
+
+ Example of the SMTP Procedure
+
+ This SMTP example shows mail sent by Smith at host Alpha.ARPA,
+ to Jones, Green, and Brown at host Beta.ARPA. Here we assume
+ that host Alpha contacts host Beta directly.
+
+ S: MAIL FROM:<Smith@Alpha.ARPA>
+ R: 250 OK
+
+ S: RCPT TO:<Jones@Beta.ARPA>
+ R: 250 OK
+
+ S: RCPT TO:<Green@Beta.ARPA>
+ R: 550 No such user here
+
+ S: RCPT TO:<Brown@Beta.ARPA>
+ R: 250 OK
+
+ S: DATA
+ R: 354 Start mail input; end with <CRLF>.<CRLF>
+ S: Blah blah blah...
+ S: ...etc. etc. etc.
+ S: <CRLF>.<CRLF>
+ R: 250 OK
+
+ The mail has now been accepted for Jones and Brown. Green did
+ not have a mailbox at host Beta.
+
+ Example 1
+
+ -------------------------------------------------------------
+
+
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+[Page 6] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+ 3.2. FORWARDING
+
+ There are some cases where the destination information in the
+ <forward-path> is incorrect, but the receiver-SMTP knows the
+ correct destination. In such cases, one of the following replies
+ should be used to allow the sender to contact the correct
+ destination.
+
+ 251 User not local; will forward to <forward-path>
+
+ This reply indicates that the receiver-SMTP knows the user's
+ mailbox is on another host and indicates the correct
+ forward-path to use in the future. Note that either the
+ host or user or both may be different. The receiver takes
+ responsibility for delivering the message.
+
+ 551 User not local; please try <forward-path>
+
+ This reply indicates that the receiver-SMTP knows the user's
+ mailbox is on another host and indicates the correct
+ forward-path to use. Note that either the host or user or
+ both may be different. The receiver refuses to accept mail
+ for this user, and the sender must either redirect the mail
+ according to the information provided or return an error
+ response to the originating user.
+
+ Example 2 illustrates the use of these responses.
+
+ -------------------------------------------------------------
+
+ Example of Forwarding
+
+ Either
+
+ S: RCPT TO:<Postel@USC-ISI.ARPA>
+ R: 251 User not local; will forward to <Postel@USC-ISIF.ARPA>
+
+ Or
+
+ S: RCPT TO:<Paul@USC-ISIB.ARPA>
+ R: 551 User not local; please try <Mockapetris@USC-ISIF.ARPA>
+
+ Example 2
+
+ -------------------------------------------------------------
+
+
+
+
+Postel [Page 7]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+ 3.3. VERIFYING AND EXPANDING
+
+ SMTP provides as additional features, commands to verify a user
+ name or expand a mailing list. This is done with the VRFY and
+ EXPN commands, which have character string arguments. For the
+ VRFY command, the string is a user name, and the response may
+ include the full name of the user and must include the mailbox of
+ the user. For the EXPN command, the string identifies a mailing
+ list, and the multiline response may include the full name of the
+ users and must give the mailboxes on the mailing list.
+
+ "User name" is a fuzzy term and used purposely. If a host
+ implements the VRFY or EXPN commands then at least local mailboxes
+ must be recognized as "user names". If a host chooses to
+ recognize other strings as "user names" that is allowed.
+
+ 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 of one mailbox. If a request is made to verify a mailing
+ list a positive response can be given if on receipt of a message
+ so addressed it will be delivered to everyone on the list,
+ otherwise an error should be reported (e.g., "550 That is a
+ mailing list, not a user"). If a request is made to expand a user
+ name a positive response can be formed by returning a list
+ containing one name, or an error can be reported (e.g., "550 That
+ is a user name, not a mailing list").
+
+ In the case of a multiline reply (normal for EXPN) exactly one
+ mailbox is to be specified on each line of the reply. In the case
+ of an ambiguous request, for example, "VRFY Smith", where there
+ are two Smith's the response must be "553 User ambiguous".
+
+ The case of verifying a user name is straightforward as shown in
+ example 3.
+
+
+
+
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+
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+
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+
+[Page 8] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+ -------------------------------------------------------------
+
+ Example of Verifying a User Name
+
+ Either
+
+ S: VRFY Smith
+ R: 250 Fred Smith <Smith@USC-ISIF.ARPA>
+
+ Or
+
+ S: VRFY Smith
+ R: 251 User not local; will forward to <Smith@USC-ISIQ.ARPA>
+
+ Or
+
+ S: VRFY Jones
+ R: 550 String does not match anything.
+
+ Or
+
+ S: VRFY Jones
+ R: 551 User not local; please try <Jones@USC-ISIQ.ARPA>
+
+ Or
+
+ S: VRFY Gourzenkyinplatz
+ R: 553 User ambiguous.
+
+ Example 3
+
+ -------------------------------------------------------------
+
+
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+Postel [Page 9]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+ The case of expanding a mailbox list requires a multiline reply as
+ shown in example 4.
+
+ -------------------------------------------------------------
+
+ Example of Expanding a Mailing List
+
+ Either
+
+ S: EXPN Example-People
+ R: 250-Jon Postel <Postel@USC-ISIF.ARPA>
+ R: 250-Fred Fonebone <Fonebone@USC-ISIQ.ARPA>
+ R: 250-Sam Q. Smith <SQSmith@USC-ISIQ.ARPA>
+ R: 250-Quincy Smith <@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>
+ R: 250-<joe@foo-unix.ARPA>
+ R: 250 <xyz@bar-unix.ARPA>
+
+ Or
+
+ S: EXPN Executive-Washroom-List
+ R: 550 Access Denied to You.
+
+ Example 4
+
+ -------------------------------------------------------------
+
+ 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 is
+ a variety of file naming conventions in the Internet.
+
+ The VRFY and EXPN commands are not included in the minimum
+ implementation (Section 4.5.1), and are not required to work
+ across relays when they are implemented.
+
+
+
+
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+
+
+
+
+
+
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+
+[Page 10] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+ 3.4. SENDING AND MAILING
+
+ The main purpose of SMTP is to deliver messages to user's
+ mailboxes. A very similar service provided by some hosts is to
+ deliver messages to user's terminals (provided the user is active
+ on the host). The delivery to the user's mailbox is called
+ "mailing", the delivery to the user's terminal is called
+ "sending". Because in many hosts the implementation of sending is
+ nearly identical to the implementation of mailing these two
+ functions are combined in SMTP. However the sending commands are
+ not included in the required minimum implementation
+ (Section 4.5.1). Users should have the ability to control the
+ writing of messages on their terminals. Most hosts permit the
+ users to accept or refuse such messages.
+
+ The following three command are defined to support the sending
+ options. These are used in the mail transaction instead of the
+ MAIL command and inform the receiver-SMTP of the special semantics
+ of this transaction:
+
+ SEND <SP> FROM:<reverse-path> <CRLF>
+
+ The SEND command requires that the mail data be delivered to
+ the user's terminal. If the user is not active (or not
+ accepting terminal messages) on the host a 450 reply may
+ returned to a RCPT command. The mail transaction is
+ successful if the message is delivered the terminal.
+
+ SOML <SP> FROM:<reverse-path> <CRLF>
+
+ The Send Or MaiL command requires that the mail data be
+ delivered to the user's terminal if the user is active (and
+ accepting terminal messages) on the host. If the user is
+ not active (or not accepting terminal messages) then the
+ mail data is entered into the user's mailbox. The mail
+ transaction is successful if the message is delivered either
+ to the terminal or the mailbox.
+
+ SAML <SP> FROM:<reverse-path> <CRLF>
+
+ The Send And MaiL command requires that the mail data be
+ delivered to the user's terminal if the user is active (and
+ accepting terminal messages) on the host. In any case the
+ mail data is entered into the user's mailbox. The mail
+ transaction is successful if the message is delivered the
+ mailbox.
+
+
+
+Postel [Page 11]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+ The same reply codes that are used for the MAIL commands are used
+ for these commands.
+
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+[Page 12] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+ 3.5. OPENING AND CLOSING
+
+ At the time the transmission channel is opened there is an
+ exchange to ensure that the hosts are communicating with the hosts
+ they think they are.
+
+ The following two commands are used in transmission channel
+ opening and closing:
+
+ HELO <SP> <domain> <CRLF>
+
+ QUIT <CRLF>
+
+ In the HELO command the host sending the command identifies
+ itself; the command may be interpreted as saying "Hello, I am
+ <domain>".
+
+ -------------------------------------------------------------
+
+ Example of Connection Opening
+
+ R: 220 BBN-UNIX.ARPA Simple Mail Transfer Service Ready
+ S: HELO USC-ISIF.ARPA
+ R: 250 BBN-UNIX.ARPA
+
+ Example 5
+
+ -------------------------------------------------------------
+
+ -------------------------------------------------------------
+
+ Example of Connection Closing
+
+ S: QUIT
+ R: 221 BBN-UNIX.ARPA Service closing transmission channel
+
+ Example 6
+
+ -------------------------------------------------------------
+
+
+
+
+
+
+
+
+
+
+Postel [Page 13]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+ 3.6. RELAYING
+
+ The forward-path may be a source route of the form
+ "@ONE,@TWO:JOE@THREE", where ONE, TWO, and THREE are hosts. 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.
+
+ Conceptually the elements of the forward-path are moved to the
+ reverse-path as the message is relayed from one server-SMTP to
+ another. The reverse-path is a reverse source route, (i.e., a
+ source route from the current location of the message to the
+ originator of the message). When a server-SMTP deletes its
+ identifier from the forward-path and inserts it into the
+ reverse-path, it must use the name it is known by in the
+ environment it is sending into, not the environment the mail came
+ from, in case the server-SMTP is known by different names in
+ different environments.
+
+ If when the message arrives at an SMTP the first element of the
+ forward-path is not the identifier of that SMTP the element is not
+ deleted from the forward-path and is used to determine the next
+ SMTP to send the message to. In any case, the SMTP adds its own
+ identifier to the reverse-path.
+
+ Using source routing the receiver-SMTP receives mail to be relayed
+ to another server-SMTP The receiver-SMTP may accept or reject the
+ task of relaying the mail in the same way it accepts or rejects
+ mail for a local user. The receiver-SMTP transforms the command
+ arguments by moving its own identifier from the forward-path to
+ the beginning of the reverse-path. The receiver-SMTP then becomes
+ a sender-SMTP, establishes a transmission channel to the next SMTP
+ in the forward-path, and sends it the mail.
+
+ The first host in the reverse-path should be the host sending the
+ SMTP commands, and the first host in the forward-path should be
+ the host receiving the SMTP commands.
+
+ 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.
+
+ If a server-SMTP has accepted the task of relaying the mail and
+
+
+
+[Page 14] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+ later finds that the forward-path is incorrect or that the mail
+ cannot be delivered for whatever 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).
+
+ This notification message must be from the server-SMTP at this
+ host. Of course, server-SMTPs should not send notification
+ messages about problems with 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 relayed it is permissible to leave the reverse-path
+ null. A MAIL command with a null reverse-path appears as follows:
+
+ MAIL FROM:<>
+
+ An undeliverable mail notification message is shown in example 7.
+ This notification is in response to a message originated by JOE at
+ HOSTW and sent via HOSTX to HOSTY with instructions to relay it on
+ to HOSTZ. What we see in the example is the transaction between
+ HOSTY and HOSTX, which is the first step in the return of the
+ notification message.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Postel [Page 15]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+ -------------------------------------------------------------
+
+ Example Undeliverable Mail Notification Message
+
+ S: MAIL FROM:<>
+ R: 250 ok
+ S: RCPT TO:<@HOSTX.ARPA:JOE@HOSTW.ARPA>
+ R: 250 ok
+ S: DATA
+ R: 354 send the mail data, end with .
+ S: Date: 23 Oct 81 11:22:33
+ S: From: SMTP@HOSTY.ARPA
+ S: To: JOE@HOSTW.ARPA
+ S: Subject: Mail System Problem
+ S:
+ S: Sorry JOE, your message to SAM@HOSTZ.ARPA lost.
+ S: HOSTZ.ARPA said this:
+ S: "550 No Such User"
+ S: .
+ R: 250 ok
+
+ Example 7
+
+ -------------------------------------------------------------
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+[Page 16] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+ 3.7. DOMAINS
+
+ Domains are a recently introduced concept in the ARPA Internet
+ mail system. The use of domains changes the address space from a
+ flat global space of simple character string host names to a
+ hierarchically structured rooted tree of global addresses. The
+ host name is replaced by a domain and host designator which is a
+ sequence of domain element strings separated by periods with the
+ understanding that the domain elements are ordered from the most
+ specific to the most general.
+
+ For example, "USC-ISIF.ARPA", "Fred.Cambridge.UK", and
+ "PC7.LCS.MIT.ARPA" might be host-and-domain identifiers.
+
+ Whenever domain names are used in SMTP only the official names are
+ used, the use of nicknames or aliases is not allowed.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Postel [Page 17]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+ 3.8. CHANGING ROLES
+
+ The TURN command may be used to reverse the roles of the two
+ programs communicating over the transmission channel.
+
+ If program-A is currently the sender-SMTP and it sends the TURN
+ command and receives an ok reply (250) then program-A becomes the
+ receiver-SMTP.
+
+ If program-B is currently the receiver-SMTP and it receives the
+ TURN command and sends an ok reply (250) then program-B becomes
+ the sender-SMTP.
+
+ To refuse to change roles the receiver sends the 502 reply.
+
+ Please note that this command is optional. It would not normally
+ be used in situations where the transmission channel is TCP.
+ However, when the cost of establishing the transmission channel is
+ high, this command may be quite useful. For example, this command
+ may be useful in supporting be mail exchange using the public
+ switched telephone system as a transmission channel, especially if
+ some hosts poll other hosts for mail exchanges.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+[Page 18] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+4. THE SMTP SPECIFICATIONS
+
+ 4.1. SMTP COMMANDS
+
+ 4.1.1. COMMAND SEMANTICS
+
+ 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 command codes themselves are
+ alphabetic characters terminated by <SP> if parameters follow
+ and <CRLF> otherwise. The syntax of mailboxes must conform to
+ receiver site conventions. The SMTP commands are discussed
+ below. The SMTP replies are discussed in the 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.
+
+ HELLO (HELO)
+
+ This command is used to identify the sender-SMTP to the
+ receiver-SMTP. The argument field contains the host name of
+ the sender-SMTP.
+
+ The receiver-SMTP identifies itself to the sender-SMTP in
+ the connection greeting reply, and in the response to this
+ command.
+
+ This command and an OK reply to it confirm that both the
+ sender-SMTP and the receiver-SMTP are in the initial state,
+ that is, there is no transaction in progress and all state
+ tables and buffers are cleared.
+
+
+
+
+
+
+
+Postel [Page 19]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+ MAIL (MAIL)
+
+ This command is used to initiate a mail transaction in which
+ the mail data is delivered to one or more mailboxes. The
+ argument field contains a reverse-path.
+
+ The reverse-path consists of an optional list of hosts and
+ the sender mailbox. 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 IPCE to which it is
+ relaying the mail rather than the IPCE from which the mail
+ came (if they are different). In some types of error
+ reporting messages (for example, undeliverable mail
+ notifications) the reverse-path may be null (see Example 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.
+
+ 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 forward-path consists of an optional list of hosts and a
+ required destination mailbox. When the list of hosts is
+ present, it is a source route and indicates that the mail
+ must be relayed to the next host on the list. If the
+ receiver-SMTP does not implement the relay function it may
+ user the same reply it would for an unknown local user
+ (550).
+
+ When mail is relayed, the relay host must remove itself from
+ the beginning forward-path and put itself at the beginning
+ of the reverse-path. When mail reaches its ultimate
+ destination (the forward-path contains only a destination
+ mailbox), the receiver-SMTP inserts it into the destination
+ mailbox in accordance with its host mail conventions.
+
+
+
+
+
+[Page 20] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+ For example, mail received at relay host A with arguments
+
+ FROM:<USERX@HOSTY.ARPA>
+ TO:<@HOSTA.ARPA,@HOSTB.ARPA:USERC@HOSTD.ARPA>
+
+ will be relayed on to host B with arguments
+
+ FROM:<@HOSTA.ARPA:USERX@HOSTY.ARPA>
+ TO:<@HOSTB.ARPA:USERC@HOSTD.ARPA>.
+
+ This command causes its forward-path argument to be appended
+ to the forward-path buffer.
+
+ DATA (DATA)
+
+ The receiver treats the lines following the command as mail
+ data from the sender. This command causes the mail data
+ from this command to be appended to the mail data buffer.
+ The mail data may contain any of the 128 ASCII character
+ codes.
+
+ The mail data is terminated by a line containing only a
+ period, that is the character sequence "<CRLF>.<CRLF>" (see
+ Section 4.5.2 on Transparency). This is the end of mail
+ data indication.
+
+ The end of mail data indication requires that the receiver
+ must now 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 completely the
+ receiver must send a failure reply.
+
+ When the receiver-SMTP accepts a message either for relaying
+ or for final delivery it inserts at the beginning of the
+ mail data a time stamp line. The time stamp line indicates
+ the identity of the host that sent the message, and 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.
+
+ When the receiver-SMTP makes the "final delivery" of a
+ message it inserts at the beginning of the mail data a
+
+
+
+Postel [Page 21]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+ return path line. The return path line preserves the
+ information in the <reverse-path> from the MAIL command.
+ Here, final delivery means the message leaves the SMTP
+ world. Normally, this would mean it has been delivered to
+ the destination user, 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 be
+ different from the actual sender's mailbox, for example,
+ if error responses are to be delivered a special error
+ handling mailbox rather than the message senders.
+
+ The preceding two paragraphs imply 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 header and body [2]. See Example 8.
+
+ Special mention is needed of the response and further action
+ required when the processing following the end of mail data
+ indication is partially successful. This could arise if
+ after accepting several recipients and the mail data, the
+ receiver-SMTP finds that the mail data can be successfully
+ delivered to some of the recipients, but it cannot be to
+ others (for example, due to mailbox space allocation
+ problems). In such a situation, the response to the DATA
+ command must be an OK reply. But, the receiver-SMTP must
+ compose and send an "undeliverable mail" notification
+ message to the originator of the message. Either a single
+ notification which lists all of the recipients that failed
+ to get the message, or separate notification messages must
+ be sent for each failed recipient (see Example 7). All
+ undeliverable mail notification messages are sent using the
+ MAIL command (even if they result from processing a SEND,
+ SOML, or SAML command).
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+[Page 22] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+ -------------------------------------------------------------
+
+ Example of Return Path and Received Time Stamps
+
+ Return-Path: <@GHI.ARPA,@DEF.ARPA,@ABC.ARPA:JOE@ABC.ARPA>
+ Received: from GHI.ARPA by JKL.ARPA ; 27 Oct 81 15:27:39 PST
+ Received: from DEF.ARPA by GHI.ARPA ; 27 Oct 81 15:15:13 PST
+ Received: from ABC.ARPA by DEF.ARPA ; 27 Oct 81 15:01:59 PST
+ Date: 27 Oct 81 15:01:01 PST
+ From: JOE@ABC.ARPA
+ Subject: Improved Mailing System Installed
+ To: SAM@JKL.ARPA
+
+ This is to inform you that ...
+
+ Example 8
+
+ -------------------------------------------------------------
+
+ SEND (SEND)
+
+ This command is used to initiate a mail transaction in which
+ the mail data is delivered to one or more terminals. The
+ argument field contains a reverse-path. This command is
+ successful if the message is delivered to a terminal.
+
+ The reverse-path consists of an optional list of hosts and
+ the sender mailbox. 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 IPCE to which it is
+ relaying the mail rather than the IPCE from which the mail
+ came (if they are different).
+
+ 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.
+
+ SEND OR MAIL (SOML)
+
+ This command is used to initiate a mail transaction in which
+ the mail data is delivered to one or more terminals or
+
+
+
+Postel [Page 23]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+ mailboxes. For each recipient the mail data is delivered to
+ the recipient's terminal if the recipient is active on the
+ host (and accepting terminal messages), otherwise to the
+ recipient's mailbox. The argument field contains a
+ reverse-path. This command is successful if the message is
+ delivered to a terminal or the mailbox.
+
+ The reverse-path consists of an optional list of hosts and
+ the sender mailbox. 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 IPCE to which it is
+ relaying the mail rather than the IPCE from which the mail
+ came (if they are different).
+
+ 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.
+
+ SEND AND MAIL (SAML)
+
+ This command is used to initiate a mail transaction in which
+ the mail data is delivered to one or more terminals and
+ mailboxes. For each recipient the mail data is delivered to
+ the recipient's terminal if the recipient is active on the
+ host (and accepting terminal messages), and for all
+ recipients to the recipient's mailbox. The argument field
+ contains a reverse-path. This command is successful if the
+ message is delivered to the mailbox.
+
+ The reverse-path consists of an optional list of hosts and
+ the sender mailbox. 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 IPCE to which it is
+ relaying the mail rather than the IPCE from which the mail
+ came (if they are different).
+
+ This command clears the reverse-path buffer, the
+
+
+
+[Page 24] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+ forward-path buffer, and the mail data buffer; and inserts
+ the reverse-path information from this command into the
+ reverse-path buffer.
+
+ RESET (RSET)
+
+ This command specifies that the current mail transaction is
+ to be aborted. Any stored sender, recipients, and mail data
+ must be discarded, and all buffers and state tables cleared.
+ The receiver must send an OK reply.
+
+ VERIFY (VRFY)
+
+ This command asks the receiver to confirm that the argument
+ identifies a user. If it is a user name, the full name of
+ the user (if known) and the fully specified mailbox are
+ returned.
+
+ This command has no effect on any of the reverse-path
+ buffer, the forward-path buffer, or the mail data buffer.
+
+ 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. The full name of the users (if
+ known) and the fully specified mailboxes are returned in a
+ multiline reply.
+
+ This command has no effect on any of the reverse-path
+ buffer, the forward-path buffer, or the mail data buffer.
+
+ HELP (HELP)
+
+ This command causes the receiver to send helpful information
+ to the sender of the HELP command. 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 any of the reverse-path
+ buffer, the forward-path buffer, or the mail data buffer.
+
+
+
+
+
+
+
+
+Postel [Page 25]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+ 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.
+
+ This command has no effect on any of the reverse-path
+ buffer, the forward-path buffer, or the mail data buffer.
+
+ QUIT (QUIT)
+
+ This command specifies that the receiver must send an OK
+ reply, and then close the transmission channel.
+
+ The receiver should not close the transmission channel until
+ it receives and replies to a QUIT command (even if there was
+ an error). The sender should not close the transmission
+ channel until it send a QUIT command and receives the reply
+ (even if there was an error response to a previous command).
+ If the connection is closed prematurely the receiver should
+ act as if a RSET command had been received (canceling any
+ pending transaction, but not undoing any previously
+ completed transaction), the sender should act as if the
+ command or transaction in progress had received a temporary
+ error (4xx).
+
+ TURN (TURN)
+
+ This command specifies that the receiver must either (1)
+ send an OK reply and then take on the role of the
+ sender-SMTP, or (2) send a refusal reply and retain the role
+ of the receiver-SMTP.
+
+ If program-A is currently the sender-SMTP and it sends the
+ TURN command and receives an OK reply (250) then program-A
+ becomes the receiver-SMTP. Program-A is then in the initial
+ state as if the transmission channel just opened, and it
+ then sends the 220 service ready greeting.
+
+ If program-B is currently the receiver-SMTP and it receives
+ the TURN command and sends an OK reply (250) then program-B
+ becomes the sender-SMTP. Program-B is then in the initial
+ state as if the transmission channel just opened, and it
+ then expects to receive the 220 service ready greeting.
+
+ To refuse to change roles the receiver sends the 502 reply.
+
+
+
+[Page 26] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+ There are restrictions on the order in which these command may
+ be used.
+
+ The first command in a session must be the HELO command.
+ The HELO command may be used later in a session as well. If
+ the HELO command argument is not acceptable a 501 failure
+ reply must be returned and the receiver-SMTP must stay in
+ the same state.
+
+ The NOOP, HELP, EXPN, and VRFY commands can be used at any
+ time during a session.
+
+ The MAIL, SEND, SOML, or SAML commands begin a mail
+ transaction. Once started a mail transaction consists of
+ one of the transaction beginning commands, one or more RCPT
+ commands, and a DATA command, in that order. A mail
+ transaction may be aborted by the RSET command. There may
+ be zero or more transactions in a session.
+
+ If the transaction beginning command argument is not
+ acceptable a 501 failure reply must be returned and the
+ receiver-SMTP must stay in the same state. If the commands
+ in a transaction are out of order a 503 failure reply must
+ be returned and the receiver-SMTP must stay in the same
+ state.
+
+ The last command in a session must be the QUIT command. The
+ QUIT command can not be used at any other time in a session.
+
+ 4.1.2. COMMAND SYNTAX
+
+ The commands consist of a command code followed by an argument
+ field. Command codes are four alphabetic characters. Upper
+ and lower case alphabetic characters are to be treated
+ identically. Thus, any of the following may represent the mail
+ command:
+
+ MAIL Mail mail MaIl mAIl
+
+ This also applies to any symbols representing parameter values,
+ such as "TO" or "to" for the forward-path. Command codes and
+ the argument fields are separated by one or more spaces.
+ However, within the reverse-path and forward-path arguments
+ case is important. In particular, in some hosts the user
+ "smith" is different from the user "Smith".
+
+
+
+
+Postel [Page 27]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+ The argument field consists of a variable length character
+ string ending with the character sequence <CRLF>. The receiver
+ is to take no action until this sequence is received.
+
+ Square brackets denote an optional argument field. If the
+ option is not taken, the appropriate default is implied.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+[Page 28] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+ The following are the SMTP commands:
+
+ HELO <SP> <domain> <CRLF>
+
+ MAIL <SP> FROM:<reverse-path> <CRLF>
+
+ RCPT <SP> TO:<forward-path> <CRLF>
+
+ DATA <CRLF>
+
+ RSET <CRLF>
+
+ SEND <SP> FROM:<reverse-path> <CRLF>
+
+ SOML <SP> FROM:<reverse-path> <CRLF>
+
+ SAML <SP> FROM:<reverse-path> <CRLF>
+
+ VRFY <SP> <string> <CRLF>
+
+ EXPN <SP> <string> <CRLF>
+
+ HELP [<SP> <string>] <CRLF>
+
+ NOOP <CRLF>
+
+ QUIT <CRLF>
+
+ TURN <CRLF>
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Postel [Page 29]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+ The syntax of the above argument fields (using BNF notation
+ where applicable) is given below. The "..." notation indicates
+ that a field may be repeated one or more times.
+
+ <reverse-path> ::= <path>
+
+ <forward-path> ::= <path>
+
+ <path> ::= "<" [ <a-d-l> ":" ] <mailbox> ">"
+
+ <a-d-l> ::= <at-domain> | <at-domain> "," <a-d-l>
+
+ <at-domain> ::= "@" <domain>
+
+ <domain> ::= <element> | <element> "." <domain>
+
+ <element> ::= <name> | "#" <number> | "[" <dotnum> "]"
+
+ <mailbox> ::= <local-part> "@" <domain>
+
+ <local-part> ::= <dot-string> | <quoted-string>
+
+ <name> ::= <a> <ldh-str> <let-dig>
+
+ <ldh-str> ::= <let-dig-hyp> | <let-dig-hyp> <ldh-str>
+
+ <let-dig> ::= <a> | <d>
+
+ <let-dig-hyp> ::= <a> | <d> | "-"
+
+ <dot-string> ::= <string> | <string> "." <dot-string>
+
+ <string> ::= <char> | <char> <string>
+
+ <quoted-string> ::= """ <qtext> """
+
+ <qtext> ::= "\" <x> | "\" <x> <qtext> | <q> | <q> <qtext>
+
+ <char> ::= <c> | "\" <x>
+
+ <dotnum> ::= <snum> "." <snum> "." <snum> "." <snum>
+
+ <number> ::= <d> | <d> <number>
+
+ <CRLF> ::= <CR> <LF>
+
+
+
+
+[Page 30] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+ <CR> ::= the carriage return character (ASCII code 13)
+
+ <LF> ::= the line feed character (ASCII code 10)
+
+ <SP> ::= the space character (ASCII code 32)
+
+ <snum> ::= one, two, or three digits representing a decimal
+ integer value in the range 0 through 255
+
+ <a> ::= any one of the 52 alphabetic characters A through Z
+ in upper case and a through z in lower case
+
+ <c> ::= any one of the 128 ASCII characters, but not any
+ <special> or <SP>
+
+ <d> ::= any one of the ten digits 0 through 9
+
+ <q> ::= any one of the 128 ASCII characters except <CR>,
+ <LF>, quote ("), or backslash (\)
+
+ <x> ::= any one of the 128 ASCII characters (no exceptions)
+
+ <special> ::= "<" | ">" | "(" | ")" | "[" | "]" | "\" | "."
+ | "," | ";" | ":" | "@" """ | the control
+ characters (ASCII codes 0 through 31 inclusive and
+ 127)
+
+ 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" could be used to indicate a single nine character
+ user field with comma being the fourth character of the field.
+
+ Hosts are generally known by names which are translated to
+ addresses in each host. Note that the name elements of domains
+ are the official names -- no use of nicknames or aliases is
+ allowed.
+
+ Sometimes a host is not known to the translation function and
+ communication is blocked. To bypass this barrier two numeric
+ forms are also allowed for host "names". One form is a decimal
+ integer prefixed by a pound sign, "#", which indicates the
+ number is the address of the host. Another form is four small
+ decimal integers separated by dots and enclosed by brackets,
+ e.g., "[123.255.37.2]", which indicates a 32-bit ARPA Internet
+ Address in four 8-bit fields.
+
+
+
+Postel [Page 31]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+ The time stamp line and the return path line are formally
+ defined as follows:
+
+ <return-path-line> ::= "Return-Path:" <SP><reverse-path><CRLF>
+
+ <time-stamp-line> ::= "Received:" <SP> <stamp> <CRLF>
+
+ <stamp> ::= <from-domain> <by-domain> <opt-info> ";"
+ <daytime>
+
+ <from-domain> ::= "FROM" <SP> <domain> <SP>
+
+ <by-domain> ::= "BY" <SP> <domain> <SP>
+
+ <opt-info> ::= [<via>] [<with>] [<id>] [<for>]
+
+ <via> ::= "VIA" <SP> <link> <SP>
+
+ <with> ::= "WITH" <SP> <protocol> <SP>
+
+ <id> ::= "ID" <SP> <string> <SP>
+
+ <for> ::= "FOR" <SP> <path> <SP>
+
+ <link> ::= The standard names for links are registered with
+ the Network Information Center.
+
+ <protocol> ::= The standard names for protocols are
+ registered with the Network Information Center.
+
+ <daytime> ::= <SP> <date> <SP> <time>
+
+ <date> ::= <dd> <SP> <mon> <SP> <yy>
+
+ <time> ::= <hh> ":" <mm> ":" <ss> <SP> <zone>
+
+ <dd> ::= the one or two decimal integer day of the month in
+ the range 1 to 31.
+
+ <mon> ::= "JAN" | "FEB" | "MAR" | "APR" | "MAY" | "JUN" |
+ "JUL" | "AUG" | "SEP" | "OCT" | "NOV" | "DEC"
+
+ <yy> ::= the two decimal integer year of the century in the
+ range 00 to 99.
+
+
+
+
+
+[Page 32] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+ <hh> ::= the two decimal integer hour of the day in the
+ range 00 to 24.
+
+ <mm> ::= the two decimal integer minute of the hour in the
+ range 00 to 59.
+
+ <ss> ::= the two decimal integer second of the minute in the
+ range 00 to 59.
+
+ <zone> ::= "UT" for Universal Time (the default) or other
+ time zone designator (as in [2]).
+
+
+
+ -------------------------------------------------------------
+
+ Return Path Example
+
+ Return-Path: <@CHARLIE.ARPA,@BAKER.ARPA:JOE@ABLE.ARPA>
+
+ Example 9
+
+ -------------------------------------------------------------
+
+ -------------------------------------------------------------
+
+ Time Stamp Line Example
+
+ Received: FROM ABC.ARPA BY XYZ.ARPA ; 22 OCT 81 09:23:59 PDT
+
+ Received: from ABC.ARPA by XYZ.ARPA via TELENET with X25
+ id M12345 for Smith@PDQ.ARPA ; 22 OCT 81 09:23:59 PDT
+
+ Example 10
+
+ -------------------------------------------------------------
+
+
+
+
+
+
+
+
+
+
+
+
+
+Postel [Page 33]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+ 4.2. SMTP REPLIES
+
+ Replies to SMTP commands are devised to ensure the synchronization
+ of requests and actions in the process of mail transfer, and to
+ guarantee that the sender-SMTP always knows the state of the
+ receiver-SMTP. Every command must generate exactly one reply.
+
+ The details of the command-reply sequence are made explicit in
+ Section 5.3 on Sequencing and Section 5.4 State Diagrams.
+
+ An SMTP reply consists of a three digit number (transmitted as
+ three alphanumeric characters) followed by some text. The number
+ is intended for use by automata to determine what state to enter
+ next; the text is meant for the human user. It is intended that
+ the three digits contain enough encoded information that the
+ sender-SMTP need not examine the text and may either discard it or
+ pass it on to the user, as appropriate. In particular, 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 Appendix E. 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
+ Appendix E). Only the EXPN and HELP commands are expected to
+ result in multiline replies in normal circumstances, however
+ multiline replies are allowed for any command.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+[Page 34] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+ 4.2.1. 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
+ 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]
+
+ 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>
+ 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]
+ 451 Requested action aborted: error in processing
+ 551 User not local; please try <forward-path>
+ 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
+
+
+
+
+
+
+
+
+
+
+
+
+
+Postel [Page 35]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+ 4.2.2. NUMERIC ORDER LIST OF REPLY CODES
+
+ 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>
+
+ 354 Start mail input; end with <CRLF>.<CRLF>
+
+ 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
+ 503 Bad sequence of commands
+ 504 Command parameter not implemented
+ 550 Requested action not taken: mailbox unavailable
+ [E.g., mailbox not found, no access]
+ 551 User not local; please try <forward-path>
+ 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
+
+
+
+
+
+
+
+
+
+
+
+
+
+[Page 36] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+ 4.3. SEQUENCING OF COMMANDS AND REPLIES
+
+ The communication between the sender and receiver is intended to
+ be an alternating dialogue, controlled by the sender. As such,
+ the sender issues a command and the receiver responds with a
+ reply. 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 of
+ the server host as the first word following the reply code.
+
+ For example,
+
+ 220 <SP> USC-ISIF.ARPA <SP> Service ready <CRLF>
+
+ The table below lists alternative success and failure replies for
+ each command. These must 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.
+
+ COMMAND-REPLY SEQUENCES
+
+ Each command is listed with its possible replies. The prefixes
+ used before the possible replies are "P" for preliminary (not
+ used in SMTP), "I" for intermediate, "S" for success, "F" for
+ failure, and "E" for error. The 421 reply (service not
+ available, closing transmission channel) may be given to any
+ command if the SMTP-receiver knows it must shut down. This
+ listing forms the basis for the State Diagrams in Section 4.4.
+
+ CONNECTION ESTABLISHMENT
+ S: 220
+ F: 421
+ HELO
+ S: 250
+ E: 500, 501, 504, 421
+ MAIL
+ S: 250
+ F: 552, 451, 452
+ E: 500, 501, 421
+
+
+
+Postel [Page 37]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+ RCPT
+ S: 250, 251
+ F: 550, 551, 552, 553, 450, 451, 452
+ E: 500, 501, 503, 421
+ DATA
+ I: 354 -> data -> S: 250
+ F: 552, 554, 451, 452
+ F: 451, 554
+ E: 500, 501, 503, 421
+ RSET
+ S: 250
+ E: 500, 501, 504, 421
+ SEND
+ S: 250
+ F: 552, 451, 452
+ E: 500, 501, 502, 421
+ SOML
+ S: 250
+ F: 552, 451, 452
+ E: 500, 501, 502, 421
+ SAML
+ S: 250
+ F: 552, 451, 452
+ E: 500, 501, 502, 421
+ VRFY
+ S: 250, 251
+ F: 550, 551, 553
+ E: 500, 501, 502, 504, 421
+ EXPN
+ S: 250
+ F: 550
+ E: 500, 501, 502, 504, 421
+ HELP
+ S: 211, 214
+ E: 500, 501, 502, 504, 421
+ NOOP
+ S: 250
+ E: 500, 421
+ QUIT
+ S: 221
+ E: 500
+ TURN
+ S: 250
+ F: 502
+ E: 500, 503
+
+
+
+
+[Page 38] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+ 4.4. STATE DIAGRAMS
+
+ Following are state diagrams for a simple-minded SMTP
+ implementation. Only the first digit of the reply codes is used.
+ There is one state diagram for each group of SMTP commands. The
+ command groupings were determined by constructing a model for each
+ command and then collecting together the commands with
+ structurally identical models.
+
+ For each command there are three possible outcomes: "success"
+ (S), "failure" (F), and "error" (E). In the state diagrams below
+ we use the symbol B for "begin", and the symbol W for "wait for
+ reply".
+
+ First, the diagram that represents most of the SMTP commands:
+
+
+ 1,3 +---+
+ ----------->| E |
+ | +---+
+ |
+ +---+ cmd +---+ 2 +---+
+ | B |---------->| W |---------->| S |
+ +---+ +---+ +---+
+ |
+ | 4,5 +---+
+ ----------->| F |
+ +---+
+
+
+ This diagram models the commands:
+
+ HELO, MAIL, RCPT, RSET, SEND, SOML, SAML, VRFY, EXPN, HELP,
+ NOOP, QUIT, TURN.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Postel [Page 39]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+ A more complex diagram models the DATA command:
+
+
+ +---+ DATA +---+ 1,2 +---+
+ | B |---------->| W |-------------------->| E |
+ +---+ +---+ ------------>+---+
+ 3| |4,5 |
+ | | |
+ -------------- ----- |
+ | | | +---+
+ | ---------- -------->| S |
+ | | | | +---+
+ | | ------------
+ | | | |
+ V 1,3| |2 |
+ +---+ data +---+ --------------->+---+
+ | |---------->| W | | F |
+ +---+ +---+-------------------->+---+
+ 4,5
+
+
+ Note that the "data" here is a series of lines sent from the
+ sender to the receiver with no response expected until the last
+ line is sent.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+[Page 40] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+ 4.5. DETAILS
+
+ 4.5.1. MINIMUM IMPLEMENTATION
+
+ In order to make SMTP workable, the following minimum
+ implementation is required for all receivers:
+
+ COMMANDS -- HELO
+ MAIL
+ RCPT
+ DATA
+ RSET
+ NOOP
+ QUIT
+
+ 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 allow all user composed text to be
+ transmitted transparently the following procedures are used.
+
+ 1. Before sending a line of mail text the sender-SMTP checks
+ the first character of the line. If it is a period, one
+ additional period is inserted at the beginning of the line.
+
+ 2. When a line of mail text is received by the receiver-SMTP
+ it checks the line. If the line is composed of a single
+ period it is the end of mail. 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 format effectors and other control characters. If
+ the transmission channel provides an 8-bit byte (octets) data
+ stream, the 7-bit ASCII codes are transmitted right justified
+ in the octets with the high order bits cleared to zero.
+
+ 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, or that store data in records rather than
+
+
+
+
+
+Postel [Page 41]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+ strings. If such transforms are necessary, they must be
+ reversible -- especially if such transforms are applied to
+ mail being relayed.
+
+ 4.5.3. SIZES
+
+ There are several objects that have required minimum maximum
+ sizes. That is, every implementation must be able to receive
+ objects of at least these sizes, but must not send objects
+ larger than these sizes.
+
+
+ ****************************************************
+ * *
+ * TO THE MAXIMUM EXTENT POSSIBLE, IMPLEMENTATION *
+ * TECHNIQUES WHICH IMPOSE NO LIMITS ON THE LENGTH *
+ * OF THESE OBJECTS SHOULD BE USED. *
+ * *
+ ****************************************************
+
+ user
+
+ The maximum total length of a user name is 64 characters.
+
+ domain
+
+ The maximum total length of a domain name or number is 64
+ 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.
+
+ reply line
+
+ The maximum total length of a reply line including the
+ reply code and the <CRLF> is 512 characters.
+
+
+
+
+
+[Page 42] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+ text line
+
+ The maximum total length of a text line including the
+ <CRLF> is 1000 characters (but not counting the leading
+ dot duplicated for transparency).
+
+ recipients buffer
+
+ The maximum total number of recipients that must be
+ buffered is 100 recipients.
+
+
+ ****************************************************
+ * *
+ * TO THE MAXIMUM EXTENT POSSIBLE, IMPLEMENTATION *
+ * TECHNIQUES WHICH IMPOSE NO LIMITS ON THE LENGTH *
+ * OF THESE OBJECTS SHOULD BE USED. *
+ * *
+ ****************************************************
+
+ Errors due to exceeding these limits may be reported by using
+ the reply codes, for example:
+
+ 500 Line too long.
+
+ 501 Path too long
+
+ 552 Too many recipients.
+
+ 552 Too much mail data.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Postel [Page 43]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+APPENDIX A
+
+ TCP Transport service
+
+ The Transmission Control Protocol [3] is used in the ARPA
+ Internet, and in any network following the US DoD standards for
+ internetwork protocols.
+
+ Connection Establishment
+
+ The SMTP transmission channel is a TCP connection established
+ between the sender process port U and the receiver process port
+ L. This single full duplex connection is used as the
+ transmission channel. This protocol is assigned the service
+ port 25 (31 octal), that is L=25.
+
+ Data Transfer
+
+ 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.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+[Page 44] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+APPENDIX B
+
+ NCP Transport service
+
+ The ARPANET Host-to-Host Protocol [4] (implemented by the Network
+ Control Program) may be used in the ARPANET.
+
+ Connection Establishment
+
+ The SMTP transmission channel is established via NCP between
+ the sender process socket U and receiver process socket L. The
+ Initial Connection Protocol [5] is followed resulting in a pair
+ of simplex connections. This pair of connections is used as
+ the transmission channel. This protocol is assigned the
+ contact socket 25 (31 octal), that is L=25.
+
+ Data Transfer
+
+ The NCP data connections are established in 8-bit byte mode.
+ 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.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Postel [Page 45]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+APPENDIX C
+
+ NITS
+
+ The Network Independent Transport Service [6] may be used.
+
+ Connection Establishment
+
+ The SMTP transmission channel is established via NITS between
+ the sender process and receiver process. The sender process
+ executes the CONNECT primitive, and the waiting receiver
+ process executes the ACCEPT primitive.
+
+ Data Transfer
+
+ The NITS 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.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+[Page 46] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+APPENDIX D
+
+ X.25 Transport service
+
+ It may be possible to use the X.25 service [7] as provided by the
+ Public Data Networks directly, however, it is suggested that a
+ reliable end-to-end protocol such as TCP be used on top of X.25
+ connections.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Postel [Page 47]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+APPENDIX E
+
+ Theory of Reply Codes
+
+ 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 sender-SMTP will be able to
+ determine its next action (proceed as planned, redo, retrench,
+ etc.) by simply examining this first digit. A sender-SMTP that
+ wants to know approximately what kind of error occurred (e.g.,
+ mail system error, command syntax error) may examine the second
+ digit, reserving the third digit 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 sender-SMTP should send
+ another command specifying whether to continue or abort
+ the action.
+
+ [Note: SMTP does not have any commands that allow this
+ type of reply, and so does not have the 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 sender-SMTP should send another command
+ specifying this information. This reply is used in
+ command sequence groups.
+
+ 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
+
+
+
+[Page 48] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+ 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 sender- SMTPs) must
+ agree on the interpretation. Each reply in this category
+ might have a different time value, but the sender-SMTP is
+ encouraged to try again. A rule of thumb to determine if
+ a reply fits into the 4yz or the 5yz category (see below)
+ is that replies are 4yz if they can be repeated without
+ any change in command form or in properties of the sender
+ or receiver. (E.g., 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 sender-SMTP 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 sender-SMTP 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 don't 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 as yet.
+
+ x4z Unspecified as yet.
+
+ 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
+
+
+
+Postel [Page 49]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+ 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.
+
+ For example, a command such as NOOP whose successful execution
+ does not offer the sender-SMTP any new information will return
+ a 250 reply. The response 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 sender-SMTP 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>.
+
+ For example:
+ 123-First line
+ 123-Second line
+ 123-234 text beginning with numbers
+ 123 The last line
+
+ In many cases the sender-SMTP then simply needs to search for
+ the reply code followed by <SP> at the beginning of a line, and
+ ignore all preceding lines. In a few cases, there is important
+ data for the sender in the reply "text". The sender will know
+ these cases from the current context.
+
+
+
+
+
+
+
+
+
+
+
+
+[Page 50] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+APPENDIX F
+
+ Scenarios
+
+ This section presents complete scenarios of several types of SMTP
+ sessions.
+
+ A Typical SMTP Transaction Scenario
+
+ This SMTP example shows mail sent by Smith at host USC-ISIF, to
+ Jones, Green, and Brown at host BBN-UNIX. Here we assume that
+ host USC-ISIF contacts host BBN-UNIX directly. The mail is
+ accepted for Jones and Brown. Green does not have a mailbox at
+ host BBN-UNIX.
+
+ -------------------------------------------------------------
+
+ R: 220 BBN-UNIX.ARPA Simple Mail Transfer Service Ready
+ S: HELO USC-ISIF.ARPA
+ R: 250 BBN-UNIX.ARPA
+
+ S: MAIL FROM:<Smith@USC-ISIF.ARPA>
+ R: 250 OK
+
+ S: RCPT TO:<Jones@BBN-UNIX.ARPA>
+ R: 250 OK
+
+ S: RCPT TO:<Green@BBN-UNIX.ARPA>
+ R: 550 No such user here
+
+ S: RCPT TO:<Brown@BBN-UNIX.ARPA>
+ R: 250 OK
+
+ S: DATA
+ R: 354 Start mail input; end with <CRLF>.<CRLF>
+ S: Blah blah blah...
+ S: ...etc. etc. etc.
+ S: .
+ R: 250 OK
+
+ S: QUIT
+ R: 221 BBN-UNIX.ARPA Service closing transmission channel
+
+ Scenario 1
+
+ -------------------------------------------------------------
+
+
+
+Postel [Page 51]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+ Aborted SMTP Transaction Scenario
+
+ -------------------------------------------------------------
+
+ R: 220 MIT-Multics.ARPA Simple Mail Transfer Service Ready
+ S: HELO ISI-VAXA.ARPA
+ R: 250 MIT-Multics.ARPA
+
+ S: MAIL FROM:<Smith@ISI-VAXA.ARPA>
+ R: 250 OK
+
+ S: RCPT TO:<Jones@MIT-Multics.ARPA>
+ R: 250 OK
+
+ S: RCPT TO:<Green@MIT-Multics.ARPA>
+ R: 550 No such user here
+
+ S: RSET
+ R: 250 OK
+
+ S: QUIT
+ R: 221 MIT-Multics.ARPA Service closing transmission channel
+
+ Scenario 2
+
+ -------------------------------------------------------------
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+[Page 52] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+ Relayed Mail Scenario
+
+ -------------------------------------------------------------
+
+ Step 1 -- Source Host to Relay Host
+
+ R: 220 USC-ISIE.ARPA Simple Mail Transfer Service Ready
+ S: HELO MIT-AI.ARPA
+ R: 250 USC-ISIE.ARPA
+
+ S: MAIL FROM:<JQP@MIT-AI.ARPA>
+ R: 250 OK
+
+ S: RCPT TO:<@USC-ISIE.ARPA:Jones@BBN-VAX.ARPA>
+ R: 250 OK
+
+ S: DATA
+ R: 354 Start mail input; end with <CRLF>.<CRLF>
+ S: Date: 2 Nov 81 22:33:44
+ S: From: John Q. Public <JQP@MIT-AI.ARPA>
+ S: Subject: The Next Meeting of the Board
+ S: To: Jones@BBN-Vax.ARPA
+ S:
+ S: Bill:
+ S: The next meeting of the board of directors will be
+ S: on Tuesday.
+ S: John.
+ S: .
+ R: 250 OK
+
+ S: QUIT
+ R: 221 USC-ISIE.ARPA Service closing transmission channel
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Postel [Page 53]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+ Step 2 -- Relay Host to Destination Host
+
+ R: 220 BBN-VAX.ARPA Simple Mail Transfer Service Ready
+ S: HELO USC-ISIE.ARPA
+ R: 250 BBN-VAX.ARPA
+
+ S: MAIL FROM:<@USC-ISIE.ARPA:JQP@MIT-AI.ARPA>
+ R: 250 OK
+
+ S: RCPT TO:<Jones@BBN-VAX.ARPA>
+ R: 250 OK
+
+ S: DATA
+ R: 354 Start mail input; end with <CRLF>.<CRLF>
+ S: Received: from MIT-AI.ARPA by USC-ISIE.ARPA ;
+ 2 Nov 81 22:40:10 UT
+ S: Date: 2 Nov 81 22:33:44
+ S: From: John Q. Public <JQP@MIT-AI.ARPA>
+ S: Subject: The Next Meeting of the Board
+ S: To: Jones@BBN-Vax.ARPA
+ S:
+ S: Bill:
+ S: The next meeting of the board of directors will be
+ S: on Tuesday.
+ S: John.
+ S: .
+ R: 250 OK
+
+ S: QUIT
+ R: 221 USC-ISIE.ARPA Service closing transmission channel
+
+ Scenario 3
+
+ -------------------------------------------------------------
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+[Page 54] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+ Verifying and Sending Scenario
+
+ -------------------------------------------------------------
+
+ R: 220 SU-SCORE.ARPA Simple Mail Transfer Service Ready
+ S: HELO MIT-MC.ARPA
+ R: 250 SU-SCORE.ARPA
+
+ S: VRFY Crispin
+ R: 250 Mark Crispin <Admin.MRC@SU-SCORE.ARPA>
+
+ S: SEND FROM:<EAK@MIT-MC.ARPA>
+ R: 250 OK
+
+ S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>
+ R: 250 OK
+
+ S: DATA
+ R: 354 Start mail input; end with <CRLF>.<CRLF>
+ S: Blah blah blah...
+ S: ...etc. etc. etc.
+ S: .
+ R: 250 OK
+
+ S: QUIT
+ R: 221 SU-SCORE.ARPA Service closing transmission channel
+
+ Scenario 4
+
+ -------------------------------------------------------------
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Postel [Page 55]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+ Sending and Mailing Scenarios
+
+ First the user's name is verified, then an attempt is made to
+ send to the user's terminal. When that fails, the messages is
+ mailed to the user's mailbox.
+
+ -------------------------------------------------------------
+
+ R: 220 SU-SCORE.ARPA Simple Mail Transfer Service Ready
+ S: HELO MIT-MC.ARPA
+ R: 250 SU-SCORE.ARPA
+
+ S: VRFY Crispin
+ R: 250 Mark Crispin <Admin.MRC@SU-SCORE.ARPA>
+
+ S: SEND FROM:<EAK@MIT-MC.ARPA>
+ R: 250 OK
+
+ S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>
+ R: 450 User not active now
+
+ S: RSET
+ R: 250 OK
+
+ S: MAIL FROM:<EAK@MIT-MC.ARPA>
+ R: 250 OK
+
+ S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>
+ R: 250 OK
+
+ S: DATA
+ R: 354 Start mail input; end with <CRLF>.<CRLF>
+ S: Blah blah blah...
+ S: ...etc. etc. etc.
+ S: .
+ R: 250 OK
+
+ S: QUIT
+ R: 221 SU-SCORE.ARPA Service closing transmission channel
+
+ Scenario 5
+
+ -------------------------------------------------------------
+
+
+
+
+
+
+[Page 56] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+ Doing the preceding scenario more efficiently.
+
+ -------------------------------------------------------------
+
+ R: 220 SU-SCORE.ARPA Simple Mail Transfer Service Ready
+ S: HELO MIT-MC.ARPA
+ R: 250 SU-SCORE.ARPA
+
+ S: VRFY Crispin
+ R: 250 Mark Crispin <Admin.MRC@SU-SCORE.ARPA>
+
+ S: SOML FROM:<EAK@MIT-MC.ARPA>
+ R: 250 OK
+
+ S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>
+ R: 250 User not active now, so will do mail.
+
+ S: DATA
+ R: 354 Start mail input; end with <CRLF>.<CRLF>
+ S: Blah blah blah...
+ S: ...etc. etc. etc.
+ S: .
+ R: 250 OK
+
+ S: QUIT
+ R: 221 SU-SCORE.ARPA Service closing transmission channel
+
+ Scenario 6
+
+ -------------------------------------------------------------
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Postel [Page 57]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+ Mailing List Scenario
+
+ First each of two mailing lists are expanded in separate sessions
+ with different hosts. Then the message is sent to everyone that
+ appeared on either list (but no duplicates) via a relay host.
+
+ -------------------------------------------------------------
+
+ Step 1 -- Expanding the First List
+
+ R: 220 MIT-AI.ARPA Simple Mail Transfer Service Ready
+ S: HELO SU-SCORE.ARPA
+ R: 250 MIT-AI.ARPA
+
+ S: EXPN Example-People
+ R: 250-<ABC@MIT-MC.ARPA>
+ R: 250-Fred Fonebone <Fonebone@USC-ISIQ.ARPA>
+ R: 250-Xenon Y. Zither <XYZ@MIT-AI.ARPA>
+ R: 250-Quincy Smith <@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>
+ R: 250-<joe@foo-unix.ARPA>
+ R: 250 <xyz@bar-unix.ARPA>
+
+ S: QUIT
+ R: 221 MIT-AI.ARPA Service closing transmission channel
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+[Page 58] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+ Step 2 -- Expanding the Second List
+
+ R: 220 MIT-MC.ARPA Simple Mail Transfer Service Ready
+ S: HELO SU-SCORE.ARPA
+ R: 250 MIT-MC.ARPA
+
+ S: EXPN Interested-Parties
+ R: 250-Al Calico <ABC@MIT-MC.ARPA>
+ R: 250-<XYZ@MIT-AI.ARPA>
+ R: 250-Quincy Smith <@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>
+ R: 250-<fred@BBN-UNIX.ARPA>
+ R: 250 <xyz@bar-unix.ARPA>
+
+ S: QUIT
+ R: 221 MIT-MC.ARPA Service closing transmission channel
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Postel [Page 59]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+ Step 3 -- Mailing to All via a Relay Host
+
+ R: 220 USC-ISIE.ARPA Simple Mail Transfer Service Ready
+ S: HELO SU-SCORE.ARPA
+ R: 250 USC-ISIE.ARPA
+
+ S: MAIL FROM:<Account.Person@SU-SCORE.ARPA>
+ R: 250 OK
+ S: RCPT TO:<@USC-ISIE.ARPA:ABC@MIT-MC.ARPA>
+ R: 250 OK
+ S: RCPT TO:<@USC-ISIE.ARPA:Fonebone@USC-ISIQA.ARPA>
+ R: 250 OK
+ S: RCPT TO:<@USC-ISIE.ARPA:XYZ@MIT-AI.ARPA>
+ R: 250 OK
+ S: RCPT
+ TO:<@USC-ISIE.ARPA,@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>
+ R: 250 OK
+ S: RCPT TO:<@USC-ISIE.ARPA:joe@FOO-UNIX.ARPA>
+ R: 250 OK
+ S: RCPT TO:<@USC-ISIE.ARPA:xyz@BAR-UNIX.ARPA>
+ R: 250 OK
+ S: RCPT TO:<@USC-ISIE.ARPA:fred@BBN-UNIX.ARPA>
+ R: 250 OK
+
+ S: DATA
+ R: 354 Start mail input; end with <CRLF>.<CRLF>
+ S: Blah blah blah...
+ S: ...etc. etc. etc.
+ S: .
+ R: 250 OK
+
+ S: QUIT
+ R: 221 USC-ISIE.ARPA Service closing transmission channel
+
+ Scenario 7
+
+ -------------------------------------------------------------
+
+
+
+
+
+
+
+
+
+
+
+
+[Page 60] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+ Forwarding Scenarios
+
+ -------------------------------------------------------------
+
+ R: 220 USC-ISIF.ARPA Simple Mail Transfer Service Ready
+ S: HELO LBL-UNIX.ARPA
+ R: 250 USC-ISIF.ARPA
+
+ S: MAIL FROM:<mo@LBL-UNIX.ARPA>
+ R: 250 OK
+
+ S: RCPT TO:<fred@USC-ISIF.ARPA>
+ R: 251 User not local; will forward to <Jones@USC-ISI.ARPA>
+
+ S: DATA
+ R: 354 Start mail input; end with <CRLF>.<CRLF>
+ S: Blah blah blah...
+ S: ...etc. etc. etc.
+ S: .
+ R: 250 OK
+
+ S: QUIT
+ R: 221 USC-ISIF.ARPA Service closing transmission channel
+
+ Scenario 8
+
+ -------------------------------------------------------------
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Postel [Page 61]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+ -------------------------------------------------------------
+
+ Step 1 -- Trying the Mailbox at the First Host
+
+ R: 220 USC-ISIF.ARPA Simple Mail Transfer Service Ready
+ S: HELO LBL-UNIX.ARPA
+ R: 250 USC-ISIF.ARPA
+
+ S: MAIL FROM:<mo@LBL-UNIX.ARPA>
+ R: 250 OK
+
+ S: RCPT TO:<fred@USC-ISIF.ARPA>
+ R: 251 User not local; will forward to <Jones@USC-ISI.ARPA>
+
+ S: RSET
+ R: 250 OK
+
+ S: QUIT
+ R: 221 USC-ISIF.ARPA Service closing transmission channel
+
+ Step 2 -- Delivering the Mail at the Second Host
+
+ R: 220 USC-ISI.ARPA Simple Mail Transfer Service Ready
+ S: HELO LBL-UNIX.ARPA
+ R: 250 USC-ISI.ARPA
+
+ S: MAIL FROM:<mo@LBL-UNIX.ARPA>
+ R: 250 OK
+
+ S: RCPT TO:<Jones@USC-ISI.ARPA>
+ R: OK
+
+ S: DATA
+ R: 354 Start mail input; end with <CRLF>.<CRLF>
+ S: Blah blah blah...
+ S: ...etc. etc. etc.
+ S: .
+ R: 250 OK
+
+ S: QUIT
+ R: 221 USC-ISI.ARPA Service closing transmission channel
+
+ Scenario 9
+
+ -------------------------------------------------------------
+
+
+
+
+[Page 62] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+ Too Many Recipients Scenario
+
+ -------------------------------------------------------------
+
+ R: 220 BERKELEY.ARPA Simple Mail Transfer Service Ready
+ S: HELO USC-ISIF.ARPA
+ R: 250 BERKELEY.ARPA
+
+ S: MAIL FROM:<Postel@USC-ISIF.ARPA>
+ R: 250 OK
+
+ S: RCPT TO:<fabry@BERKELEY.ARPA>
+ R: 250 OK
+
+ S: RCPT TO:<eric@BERKELEY.ARPA>
+ R: 552 Recipient storage full, try again in another transaction
+
+ S: DATA
+ R: 354 Start mail input; end with <CRLF>.<CRLF>
+ S: Blah blah blah...
+ S: ...etc. etc. etc.
+ S: .
+ R: 250 OK
+
+ S: MAIL FROM:<Postel@USC-ISIF.ARPA>
+ R: 250 OK
+
+ S: RCPT TO:<eric@BERKELEY.ARPA>
+ R: 250 OK
+
+ S: DATA
+ R: 354 Start mail input; end with <CRLF>.<CRLF>
+ S: Blah blah blah...
+ S: ...etc. etc. etc.
+ S: .
+ R: 250 OK
+
+ S: QUIT
+ R: 221 BERKELEY.ARPA Service closing transmission channel
+
+ Scenario 10
+
+ -------------------------------------------------------------
+
+ Note that a real implementation must handle many recipients as
+ specified in Section 4.5.3.
+
+
+
+Postel [Page 63]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+GLOSSARY
+
+ ASCII
+
+ American Standard Code for Information Interchange [1].
+
+ command
+
+ A request for a mail service action sent by the sender-SMTP to the
+ receiver-SMTP.
+
+ domain
+
+ The hierarchially structured global character string address of a
+ host computer in the mail system.
+
+ end of mail data indication
+
+ A special sequence of characters that indicates the end of the
+ mail data. In particular, the five characters carriage return,
+ line feed, period, carriage return, line feed, in that order.
+
+ host
+
+ A computer in the internetwork environment on which mailboxes or
+ SMTP processes reside.
+
+ line
+
+ A a sequence of ASCII characters ending with a <CRLF>.
+
+ mail data
+
+ A sequence of ASCII characters of arbitrary length, which conforms
+ to the standard set in the Standard for the Format of ARPA
+ Internet Text Messages (RFC 822 [2]).
+
+ mailbox
+
+ A character string (address) which identifies a user to whom mail
+ is to be sent. Mailbox normally consists of the host and user
+ specifications. The standard mailbox naming convention is defined
+ to be "user@domain". Additionally, the "container" in which mail
+ is stored.
+
+
+
+
+
+[Page 64] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+ receiver-SMTP process
+
+ A process which transfers mail in cooperation with a sender-SMTP
+ process. It waits for a connection to be established via the
+ transport service. It receives SMTP commands from the
+ sender-SMTP, sends replies, and performs the specified operations.
+
+ reply
+
+ A 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 completion code
+ (including error codes) followed by a text string. The codes are
+ for use by programs and the text is usually intended for human
+ users.
+
+ sender-SMTP process
+
+ A process which transfers mail in cooperation with a receiver-SMTP
+ process. A local language may be used in the user interface
+ command/reply dialogue. The sender-SMTP initiates the transport
+ service connection. It initiates SMTP commands, receives replies,
+ and governs the transfer of mail.
+
+ session
+
+ The set of exchanges that occur while the transmission channel is
+ open.
+
+ transaction
+
+ The set of exchanges required for one message to be transmitted
+ for one or more recipients.
+
+ transmission channel
+
+ A full-duplex communication path between a sender-SMTP and a
+ receiver-SMTP for the exchange of commands, replies, and mail
+ text.
+
+ transport service
+
+ Any reliable stream-oriented data communication services. For
+ example, NCP, TCP, NITS.
+
+
+
+
+
+Postel [Page 65]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+ user
+
+ A human being (or a process on behalf of a human being) wishing to
+ obtain mail transfer service. In addition, a recipient of
+ computer mail.
+
+ word
+
+ A sequence of printing characters.
+
+ <CRLF>
+
+ The characters carriage return and line feed (in that order).
+
+ <SP>
+
+ The space character.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+[Page 66] Postel
+
+
+
+RFC 821 August 1982
+ Simple Mail Transfer Protocol
+
+
+
+REFERENCES
+
+ [1] ASCII
+
+ ASCII, "USA Code for Information Interchange", United States of
+ America Standards Institute, X3.4, 1968. Also in: Feinler, E.
+ and J. Postel, eds., "ARPANET Protocol Handbook", NIC 7104, for
+ the Defense Communications Agency by SRI International, Menlo
+ Park, California, Revised January 1978.
+
+ [2] RFC 822
+
+ Crocker, D., "Standard for the Format of ARPA Internet Text
+ Messages," RFC 822, Department of Electrical Engineering,
+ University of Delaware, August 1982.
+
+ [3] TCP
+
+ Postel, J., ed., "Transmission Control Protocol - DARPA Internet
+ Program Protocol Specification", RFC 793, USC/Information Sciences
+ Institute, NTIS AD Number A111091, September 1981. Also in:
+ Feinler, E. and J. Postel, eds., "Internet Protocol Transition
+ Workbook", SRI International, Menlo Park, California, March 1982.
+
+ [4] NCP
+
+ McKenzie,A., "Host/Host Protocol for the ARPA Network", NIC 8246,
+ January 1972. Also in: Feinler, E. and J. Postel, eds., "ARPANET
+ Protocol Handbook", NIC 7104, for the Defense Communications
+ Agency by SRI International, Menlo Park, California, Revised
+ January 1978.
+
+ [5] Initial Connection Protocol
+
+ Postel, J., "Official Initial Connection Protocol", NIC 7101,
+ 11 June 1971. Also in: Feinler, E. and J. Postel, eds., "ARPANET
+ Protocol Handbook", NIC 7104, for the Defense Communications
+ Agency by SRI International, Menlo Park, California, Revised
+ January 1978.
+
+ [6] NITS
+
+ PSS/SG3, "A Network Independent Transport Service", Study Group 3,
+ The Post Office PSS Users Group, February 1980. Available from
+ the DCPU, National Physical Laboratory, Teddington, UK.
+
+
+
+
+Postel [Page 67]
+
+
+
+August 1982 RFC 821
+Simple Mail Transfer Protocol
+
+
+
+ [7] X.25
+
+ CCITT, "Recommendation X.25 - Interface Between Data Terminal
+ Equipment (DTE) and Data Circuit-terminating Equipment (DCE) for
+ Terminals Operating in the Packet Mode on Public Data Networks,"
+ CCITT Orange Book, Vol. VIII.2, International Telephone and
+ Telegraph Consultative Committee, Geneva, 1976.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+[Page 68] Postel
+