4051 lines
118 KiB
Plaintext
4051 lines
118 KiB
Plaintext
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RFC 821
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SIMPLE MAIL TRANSFER PROTOCOL
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Jonathan B. Postel
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August 1982
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Information Sciences Institute
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University of Southern California
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4676 Admiralty Way
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Marina del Rey, California 90291
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(213) 822-1511
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RFC 821 August 1982
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Simple Mail Transfer Protocol
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TABLE OF CONTENTS
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1. INTRODUCTION .................................................. 1
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2. THE SMTP MODEL ................................................ 2
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3. THE SMTP PROCEDURE ............................................ 4
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3.1. Mail ..................................................... 4
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3.2. Forwarding ............................................... 7
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3.3. Verifying and Expanding .................................. 8
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3.4. Sending and Mailing ..................................... 11
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3.5. Opening and Closing ..................................... 13
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3.6. Relaying ................................................ 14
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3.7. Domains ................................................. 17
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3.8. Changing Roles .......................................... 18
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4. THE SMTP SPECIFICATIONS ...................................... 19
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4.1. SMTP Commands ........................................... 19
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4.1.1. Command Semantics ..................................... 19
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4.1.2. Command Syntax ........................................ 27
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4.2. SMTP Replies ............................................ 34
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4.2.1. Reply Codes by Function Group ......................... 35
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4.2.2. Reply Codes in Numeric Order .......................... 36
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4.3. Sequencing of Commands and Replies ...................... 37
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4.4. State Diagrams .......................................... 39
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4.5. Details ................................................. 41
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4.5.1. Minimum Implementation ................................ 41
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4.5.2. Transparency .......................................... 41
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4.5.3. Sizes ................................................. 42
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APPENDIX A: TCP ................................................. 44
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APPENDIX B: NCP ................................................. 45
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APPENDIX C: NITS ................................................ 46
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APPENDIX D: X.25 ................................................ 47
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APPENDIX E: Theory of Reply Codes ............................... 48
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APPENDIX F: Scenarios ........................................... 51
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GLOSSARY ......................................................... 64
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REFERENCES ....................................................... 67
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Network Working Group J. Postel
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Request for Comments: DRAFT ISI
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Replaces: RFC 788, 780, 772 August 1982
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SIMPLE MAIL TRANSFER PROTOCOL
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1. INTRODUCTION
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The objective of Simple Mail Transfer Protocol (SMTP) is to transfer
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mail reliably and efficiently.
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SMTP is independent of the particular transmission subsystem and
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requires only a reliable ordered data stream channel. Appendices A,
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B, C, and D describe the use of SMTP with various transport services.
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A Glossary provides the definitions of terms as used in this
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document.
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An important feature of SMTP is its capability to relay mail across
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transport service environments. A transport service provides an
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interprocess communication environment (IPCE). An IPCE may cover one
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network, several networks, or a subset of a network. It is important
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to realize that transport systems (or IPCEs) are not one-to-one with
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networks. A process can communicate directly with another process
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through any mutually known IPCE. Mail is an application or use of
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interprocess communication. Mail can be communicated between
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processes in different IPCEs by relaying through a process connected
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to two (or more) IPCEs. More specifically, mail can be relayed
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between hosts on different transport systems by a host on both
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transport systems.
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Postel [Page 1]
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August 1982 RFC 821
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Simple Mail Transfer Protocol
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2. THE SMTP MODEL
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The SMTP design is based on the following model of communication: as
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the result of a user mail request, the sender-SMTP establishes a
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two-way transmission channel to a receiver-SMTP. The receiver-SMTP
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may be either the ultimate destination or an intermediate. SMTP
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commands are generated by the sender-SMTP and sent to the
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receiver-SMTP. SMTP replies are sent from the receiver-SMTP to the
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sender-SMTP in response to the commands.
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Once the transmission channel is established, the SMTP-sender sends a
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MAIL command indicating the sender of the mail. If the SMTP-receiver
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can accept mail it responds with an OK reply. The SMTP-sender then
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sends a RCPT command identifying a recipient of the mail. If the
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SMTP-receiver can accept mail for that recipient it responds with an
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OK reply; if not, it responds with a reply rejecting that recipient
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(but not the whole mail transaction). The SMTP-sender and
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SMTP-receiver may negotiate several recipients. When the recipients
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have been negotiated the SMTP-sender sends the mail data, terminating
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with a special sequence. If the SMTP-receiver successfully processes
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the mail data it responds with an OK reply. The dialog is purposely
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lock-step, one-at-a-time.
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-------------------------------------------------------------
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+----------+ +----------+
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+------+ | | | |
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| User |<-->| | SMTP | |
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+------+ | Sender- |Commands/Replies| Receiver-|
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+------+ | SMTP |<-------------->| SMTP | +------+
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| File |<-->| | and Mail | |<-->| File |
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|System| | | | | |System|
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+------+ +----------+ +----------+ +------+
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Sender-SMTP Receiver-SMTP
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Model for SMTP Use
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Figure 1
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-------------------------------------------------------------
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The SMTP provides mechanisms for the transmission of mail; directly
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from the sending user's host to the receiving user's host when the
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[Page 2] Postel
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RFC 821 August 1982
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Simple Mail Transfer Protocol
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two host are connected to the same transport service, or via one or
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more relay SMTP-servers when the source and destination hosts are not
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connected to the same transport service.
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To be able to provide the relay capability the SMTP-server must be
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supplied with the name of the ultimate destination host as well as
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the destination mailbox name.
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The argument to the MAIL command is a reverse-path, which specifies
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who the mail is from. The argument to the RCPT command is a
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forward-path, which specifies who the mail is to. The forward-path
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is a source route, while the reverse-path is a return route (which
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may be used to return a message to the sender when an error occurs
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with a relayed message).
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When the same message is sent to multiple recipients the SMTP
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encourages the transmission of only one copy of the data for all the
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recipients at the same destination host.
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The mail commands and replies have a rigid syntax. Replies also have
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a numeric code. In the following, examples appear which use actual
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commands and replies. The complete lists of commands and replies
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appears in Section 4 on specifications.
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Commands and replies are not case sensitive. That is, a command or
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reply word may be upper case, lower case, or any mixture of upper and
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lower case. Note that this is not true of mailbox user names. For
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some hosts the user name is case sensitive, and SMTP implementations
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must take case to preserve the case of user names as they appear in
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mailbox arguments. Host names are not case sensitive.
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Commands and replies are composed of characters from the ASCII
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character set [1]. When the transport service provides an 8-bit byte
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(octet) transmission channel, each 7-bit character is transmitted
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right justified in an octet with the high order bit cleared to zero.
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When specifying the general form of a command or reply, an argument
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(or special symbol) will be denoted by a meta-linguistic variable (or
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constant), for example, "<string>" or "<reverse-path>". Here the
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angle brackets indicate these are meta-linguistic variables.
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However, some arguments use the angle brackets literally. For
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example, an actual reverse-path is enclosed in angle brackets, i.e.,
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"<John.Smith@USC-ISI.ARPA>" is an instance of <reverse-path> (the
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angle brackets are actually transmitted in the command or reply).
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Postel [Page 3]
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August 1982 RFC 821
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Simple Mail Transfer Protocol
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3. THE SMTP PROCEDURES
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This section presents the procedures used in SMTP in several parts.
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First comes the basic mail procedure defined as a mail transaction.
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Following this are descriptions of forwarding mail, verifying mailbox
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names and expanding mailing lists, sending to terminals instead of or
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in combination with mailboxes, and the opening and closing exchanges.
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At the end of this section are comments on relaying, a note on mail
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domains, and a discussion of changing roles. Throughout this section
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are examples of partial command and reply sequences, several complete
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scenarios are presented in Appendix F.
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3.1. MAIL
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There are three steps to SMTP mail transactions. The transaction
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is started with a MAIL command which gives the sender
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identification. A series of one or more RCPT commands follows
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giving the receiver information. Then a DATA command gives the
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mail data. And finally, the end of mail data indicator confirms
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the transaction.
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The first step in the procedure is the MAIL command. The
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<reverse-path> contains the source mailbox.
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MAIL <SP> FROM:<reverse-path> <CRLF>
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This command tells the SMTP-receiver that a new mail
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transaction is starting and to reset all its state tables and
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buffers, including any recipients or mail data. It gives the
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reverse-path which can be used to report errors. If accepted,
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the receiver-SMTP returns a 250 OK reply.
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The <reverse-path> can contain more than just a mailbox. The
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<reverse-path> is a reverse source routing list of hosts and
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source mailbox. The first host in the <reverse-path> should be
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the host sending this command.
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The second step in the procedure is the RCPT command.
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RCPT <SP> TO:<forward-path> <CRLF>
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This command gives a forward-path identifying one recipient.
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If accepted, the receiver-SMTP returns a 250 OK reply, and
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stores the forward-path. If the recipient is unknown the
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receiver-SMTP returns a 550 Failure reply. This second step of
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the procedure can be repeated any number of times.
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[Page 4] Postel
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RFC 821 August 1982
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Simple Mail Transfer Protocol
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The <forward-path> can contain more than just a mailbox. The
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<forward-path> is a source routing list of hosts and the
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destination mailbox. The first host in the <forward-path>
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should be the host receiving this command.
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The third step in the procedure is the DATA command.
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DATA <CRLF>
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If accepted, the receiver-SMTP returns a 354 Intermediate reply
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and considers all succeeding lines to be the message text.
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When the end of text is received and stored the SMTP-receiver
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sends a 250 OK reply.
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Since the mail data is sent on the transmission channel the end
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of the mail data must be indicated so that the command and
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reply dialog can be resumed. SMTP indicates the end of the
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mail data by sending a line containing only a period. A
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transparency procedure is used to prevent this from interfering
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with the user's text (see Section 4.5.2).
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Please note that the mail data includes the memo header
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items such as Date, Subject, To, Cc, From [2].
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The end of mail data indicator also confirms the mail
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transaction and tells the receiver-SMTP to now process the
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stored recipients and mail data. If accepted, the
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receiver-SMTP returns a 250 OK reply. The DATA command should
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fail only if the mail transaction was incomplete (for example,
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no recipients), or if resources are not available.
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The above procedure is an example of a mail transaction. These
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commands must be used only in the order discussed above.
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Example 1 (below) illustrates the use of these commands in a mail
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transaction.
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Postel [Page 5]
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August 1982 RFC 821
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Simple Mail Transfer Protocol
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-------------------------------------------------------------
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Example of the SMTP Procedure
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This SMTP example shows mail sent by Smith at host Alpha.ARPA,
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to Jones, Green, and Brown at host Beta.ARPA. Here we assume
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that host Alpha contacts host Beta directly.
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S: MAIL FROM:<Smith@Alpha.ARPA>
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R: 250 OK
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S: RCPT TO:<Jones@Beta.ARPA>
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R: 250 OK
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S: RCPT TO:<Green@Beta.ARPA>
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R: 550 No such user here
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S: RCPT TO:<Brown@Beta.ARPA>
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R: 250 OK
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S: DATA
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R: 354 Start mail input; end with <CRLF>.<CRLF>
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S: Blah blah blah...
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S: ...etc. etc. etc.
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S: <CRLF>.<CRLF>
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R: 250 OK
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The mail has now been accepted for Jones and Brown. Green did
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not have a mailbox at host Beta.
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Example 1
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-------------------------------------------------------------
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[Page 6] Postel
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RFC 821 August 1982
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Simple Mail Transfer Protocol
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3.2. FORWARDING
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There are some cases where the destination information in the
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<forward-path> is incorrect, but the receiver-SMTP knows the
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correct destination. In such cases, one of the following replies
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should be used to allow the sender to contact the correct
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destination.
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251 User not local; will forward to <forward-path>
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This reply indicates that the receiver-SMTP knows the user's
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mailbox is on another host and indicates the correct
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forward-path to use in the future. Note that either the
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host or user or both may be different. The receiver takes
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responsibility for delivering the message.
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551 User not local; please try <forward-path>
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This reply indicates that the receiver-SMTP knows the user's
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mailbox is on another host and indicates the correct
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forward-path to use. Note that either the host or user or
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both may be different. The receiver refuses to accept mail
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for this user, and the sender must either redirect the mail
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according to the information provided or return an error
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response to the originating user.
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Example 2 illustrates the use of these responses.
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-------------------------------------------------------------
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Example of Forwarding
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Either
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S: RCPT TO:<Postel@USC-ISI.ARPA>
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R: 251 User not local; will forward to <Postel@USC-ISIF.ARPA>
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Or
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S: RCPT TO:<Paul@USC-ISIB.ARPA>
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R: 551 User not local; please try <Mockapetris@USC-ISIF.ARPA>
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Example 2
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-------------------------------------------------------------
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Postel [Page 7]
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August 1982 RFC 821
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Simple Mail Transfer Protocol
|
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|
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3.3. VERIFYING AND EXPANDING
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SMTP provides as additional features, commands to verify a user
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name or expand a mailing list. This is done with the VRFY and
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EXPN commands, which have character string arguments. For the
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VRFY command, the string is a user name, and the response may
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include the full name of the user and must include the mailbox of
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the user. For the EXPN command, the string identifies a mailing
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list, and the multiline response may include the full name of the
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users and must give the mailboxes on the mailing list.
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"User name" is a fuzzy term and used purposely. If a host
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implements the VRFY or EXPN commands then at least local mailboxes
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must be recognized as "user names". If a host chooses to
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recognize other strings as "user names" that is allowed.
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In some hosts the distinction between a mailing list and an alias
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for a single mailbox is a bit fuzzy, since a common data structure
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may hold both types of entries, and it is possible to have mailing
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lists of one mailbox. If a request is made to verify a mailing
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list a positive response can be given if on receipt of a message
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so addressed it will be delivered to everyone on the list,
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otherwise an error should be reported (e.g., "550 That is a
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mailing list, not a user"). If a request is made to expand a user
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name a positive response can be formed by returning a list
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containing one name, or an error can be reported (e.g., "550 That
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is a user name, not a mailing list").
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In the case of a multiline reply (normal for EXPN) exactly one
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mailbox is to be specified on each line of the reply. In the case
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of an ambiguous request, for example, "VRFY Smith", where there
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are two Smith's the response must be "553 User ambiguous".
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The case of verifying a user name is straightforward as shown in
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example 3.
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[Page 8] Postel
|
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|
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RFC 821 August 1982
|
||
Simple Mail Transfer Protocol
|
||
|
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-------------------------------------------------------------
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Example of Verifying a User Name
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Either
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S: VRFY Smith
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R: 250 Fred Smith <Smith@USC-ISIF.ARPA>
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Or
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S: VRFY Smith
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R: 251 User not local; will forward to <Smith@USC-ISIQ.ARPA>
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Or
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S: VRFY Jones
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R: 550 String does not match anything.
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Or
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S: VRFY Jones
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R: 551 User not local; please try <Jones@USC-ISIQ.ARPA>
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Or
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S: VRFY Gourzenkyinplatz
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R: 553 User ambiguous.
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Example 3
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-------------------------------------------------------------
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|
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Postel [Page 9]
|
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August 1982 RFC 821
|
||
Simple Mail Transfer Protocol
|
||
|
||
|
||
|
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The case of expanding a mailbox list requires a multiline reply as
|
||
shown in example 4.
|
||
|
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-------------------------------------------------------------
|
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|
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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.
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
[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.
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
[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
|
||
|