www.faqs.org/rfcs/rfc821.html
RFC 788, 780, 772 August 1982 SIMPLE MAIL TRANSFER PROTOCOL 1 INTRODUCTION The objective of Simple Mail Transfer Protocol (SMTP) is to transfer mail reliably and efficiently. SMTP is independent of the particular transmission subsystem and requires only a reliable ordered data stream channel. Appendices A, B, C, and D describe the use of SMTP with various transport services. A Glossary provides the definitions of terms as used in this document. An important feature of SMTP is its capability to relay mail across transport service environments. A transport service provides an interprocess communication environment (IPCE). An IPCE may cover one network, several networks, or a subset of a network. It is important to realize that transport systems (or IPCEs) are not one-to-one with networks. A process can communicate directly with another process through any mutually known IPCE. Mail is an application or use of interprocess communication. Mail can be communicated between processes in different IPCEs by relaying through a process connected to two (or more) IPCEs. More specifically, mail can be relayed between hosts on different transport systems by a host on both transport systems.
RFC 821 Simple Mail Transfer Protocol 2 THE SMTP MODEL The SMTP design is based on the following model of communication: as the result of a user mail request, the sender-SMTP establishes a two-way transmission channel to a receiver-SMTP The receiver-SMTP may be either the ultimate destination or an intermediate. SMTP commands are generated by the sender-SMTP and sent to the receiver-SMTP SMTP replies are sent from the receiver-SMTP to the sender-SMTP in response to the commands. Once the transmission channel is established, the SMTP-sender sends a MAIL command indicating the sender of the mail. If the SMTP-receiver can accept mail it responds with an OK reply. The SMTP-sender then sends a RCPT command identifying a recipient of the mail. If the SMTP-receiver can accept mail for that recipient it responds with an OK reply; if not, it responds with a reply rejecting that recipient (but not the whole mail transaction). The SMTP-sender and SMTP-receiver may negotiate several recipients. When the recipients have been negotiated the SMTP-sender sends the mail data, terminating with a special sequence. If the SMTP-receiver successfully processes the mail data it responds with an OK reply.
RFC 821 August 1982 Simple Mail Transfer Protocol two host are connected to the same transport service, or via one or more relay SMTP-servers when the source and destination hosts are not connected to the same transport service. To be able to provide the relay capability the SMTP-server must be supplied with the name of the ultimate destination host as well as the destination mailbox name. The argument to the MAIL command is a reverse-path, which specifies who the mail is from. The argument to the RCPT command is a forward-path, which specifies who the mail is to. The forward-path is a source route, while the reverse-path is a return route (which may be used to return a message to the sender when an error occurs with a relayed message). When the same message is sent to multiple recipients the SMTP encourages the transmission of only one copy of the data for all the recipients at the same destination host. In the following, examples appear which use actual commands and replies. The complete lists of commands and replies appears in Section 4 on specifications. That is, a command or reply word may be upper case, lower case, or any mixture of upper and lower case. For some hosts the user name is case sensitive, and SMTP implementations must take case to preserve the case of user names as they appear in mailbox arguments.
When the transport service provides an 8-bit byte (octet) transmission channel, each 7-bit character is transmitted right justified in an octet with the high order bit cleared to zero. When specifying the general form of a command or reply, an argument (or special symbol) will be denoted by a meta-linguistic variable (or constant), for example, "<string>" or "<reverse-path>". Here the angle brackets indicate these are meta-linguistic variables. However, some arguments use the angle brackets literally.
RFC 821 Simple Mail Transfer Protocol 3 THE SMTP PROCEDURES This section presents the procedures used in SMTP in several parts. First comes the basic mail procedure defined as a mail transaction. Following this are descriptions of forwarding mail, verifying mailbox names and expanding mailing lists, sending to terminals instead of or in combination with mailboxes, and the opening and closing exchanges. At the end of this section are comments on relaying, a note on mail domains, and a discussion of changing roles. Throughout this section are examples of partial command and reply sequences, several complete scenarios are presented in Appendix F 31 MAIL There are three steps to SMTP mail transactions. The transaction is started with a MAIL command which gives the sender identification. A series of one or more RCPT commands follows giving the receiver information. And finally, the end of mail data indicator confirms the transaction. MAIL <SP> FROM:<reverse-path> <CRLF> This command tells the SMTP-receiver that a new mail transaction is starting and to reset all its state tables and buffers, including any recipients or mail data. It gives the reverse-path which can be used to report errors. The <reverse-path> can contain more than just a mailbox. The <reverse-path> is a reverse source routing list of hosts and source mailbox. The first host in the <reverse-path> should be the host sending this command. RCPT <SP> TO:<forward-path> <CRLF> This command gives a forward-path identifying one recipient. If accepted, the receiver-SMTP returns a 250 OK reply, and stores the forward-path. If the recipient is unknown the receiver-SMTP returns a 550 Failure reply. This second step of the procedure can be repeated any number of times.
RFC 821 August 1982 Simple Mail Transfer Protocol The <forward-path> can contain more than just a mailbox. The <forward-path> is a source routing list of hosts and the destination mailbox. The first host in the <forward-path> should be the host receiving this command. DATA <CRLF> If accepted, the receiver-SMTP returns a 354 Intermediate reply and considers all succeeding lines to be the message text. When the end of text is received and stored the SMTP-receiver sends a 250 OK reply. Since the mail data is sent on the transmission channel the end of the mail data must be indicated so that the command and reply dialog can be resumed. SMTP indicates the end of the mail data by sending a line containing only a period.
The end of mail data indicator also confirms the mail transaction and tells the receiver-SMTP to now process the stored recipients and mail data. The DATA command should fail only if the mail transaction was incomplete (for example, no recipients), or if resources are not available. The above procedure is an example of a mail transaction. These commands must be used only in the order discussed above. Example 1 (below) illustrates the use of these commands in a mail transaction.
RFC 821 August 1982 Simple Mail Transfer Protocol 32 FORWARDING There are some cases where the destination information in the <forward-path> is incorrect, but the receiver-SMTP knows the correct destination. In such cases, one of the following replies should be used to allow the sender to contact the correct destination. Note that either the host or user or both may be different. The receiver takes responsibility for delivering the message. Note that either the host or user or both may be different. The receiver refuses to accept mail for this user, and the sender must either redirect the mail according to the information provided or return an error response to the originating user.
RFC 821 Simple Mail Transfer Protocol 33 VERIFYING AND EXPANDING SMTP provides as additional features, commands to verify a user name or expand a mailing list. This is done with the VRFY and EXPN commands, which have character string arguments. For the VRFY command, the string is a user name, and the...
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