An Extensive Look at SMTP Commands

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 next part.

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.

In the sub section "Basic Commands List" you will find details about all the basic SMTP commands that were defined in RFC 821.

There are restrictions on the order in which these commands 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.