RFC 5321 ● 3.3 ● Mail Transactions

There are three steps to SMTP mail transactions. The transaction starts with a MAIL command that gives the sender identification. (In general, the MAIL command may be sent only when no mail transaction is in progress; see Section 4.1.4.) A series of one or more RCPT commands follows, giving the receiver information. Then, a DATA command initiates transfer of the mail data and is terminated by the "end of mail" data indicator, which also confirms the transaction.

The first step in the procedure is the MAIL command.

MAIL FROM:<reverse-path> [SP <mail-parameters> ] <CRLF>

This command tells the SMTP-receiver that a new mail transaction is starting and to reset all its state tables and buffers, including any recipients or mail data. The <reverse-path> portion of the first or only argument contains the source mailbox (between "<" and ">" brackets), which can be used to report errors (see Section 4.2 for a discussion of error reporting). If accepted, the SMTP server returns a "250 OK" reply. If the mailbox specification is not acceptable for some reason, the server MUST return a reply indicating whether the failure is permanent (i.e., will occur again if the client tries to send the same address again) or temporary (i.e., the address might be accepted if the client tries again later). Despite the apparent scope of this requirement, there are circumstances in which the acceptability of the reverse-path may not be determined until one or more forward-paths (in RCPT commands) can be examined. In those cases, the server MAY reasonably accept the reverse-path (with a 250 reply) and then report problems after the forward-paths are received and examined. Normally, failures produce 550 or 553 replies.

Historically, the <reverse-path> was permitted to contain more than just a mailbox; however, contemporary systems SHOULD NOT use source routing (see Appendix C).

The optional <mail-parameters> are associated with negotiated SMTP service extensions (see Section 2.2).