Protocol Definition
Simple Mail Transport Protocol
SMTP RFC 821, Jonathan B. Postel, August 1982
SMTP uses TCP port 25.

Protocol Model
The SMTP design is based on the following model of communication: as the result of a user mail request, the sender-SMTP establishes a two-way transmission channel to a receiver-SMTP. The receiver-SMTP may be either the ultimate destination or an intermediate. SMTP commands are generated by the sender-SMTP and sent to the receiver-SMTP. SMTP replies are sent from the receiver-SMTP to the sender-SMTP in response to the commands.

Once the transmission channel is established, the SMTP-sender sends a MAIL command indicating the sender of the mail. If the SMTP-receiver can accept mail it responds with an OK reply. The SMTP-sender then sends a RCPT command identifying a recipient of the mail. If the SMTP-receiver can accept mail for that recipient it responds with an OK reply; if not, it responds with a reply rejecting that recipient (but not the whole mail transaction). The SMTP-sender and SMTP-receiver may negotiate several recipients. When the recipients have been negotiated the SMTP-sender sends the mail data, terminating with a special sequence. If the SMTP-receiver successfully processes the mail data it responds with an OK reply. The dialog is purposely lock-step, one-at-a-time.


Command Summary
SMTP commands are character strings terminated by <CRLF>. 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.

The command codes themselves are alphabetic characters terminated by <SP> if parameters follow and <CRLF> otherwise.

Command Syntax Error Codes
Description
HELLO HELO domain S: 250
E: 500, 501, 504, 421
Deliver mail to one or more mailboxes. The argument field contains a reverse-path.
MAIL MAIL FROM:reverse-path S: 250
F: 552, 451, 452
E: 500, 501, 421
Deliver mail to one or more mailboxes. The argument field contains a reverse-path.
RECIPIENT RCPT TO:forward-path S: 250, 251
F: 550, 551, 552, 553, 450, 451, 452
E: 500, 501, 503, 421
Set a recipient of the mail data; multiple recipients are specified by multiple uses of this command.
DATA DATA I: 354 -> data ->
-> S: 250
-> F: 552, 554, 451, 452
F: 451, 554
E: 500, 501, 503, 421
The lines following the command is mail data from the sender. The mail data from this command is 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>". This is the end of mail data indication.
SEND SEND FROM:reverse-path S: 250
F: 552, 451, 452
E: 500, 501, 502, 421
Deliver mail to one or more terminals. The argument field contains a reverse-path.
SEND OR MAIL SOML FROM:reverse-path S: 250
F: 552, 451, 452
E: 500, 501, 502, 421
Deliver mail to one or more terminals or mailboxes. The argument field contains a reverse-path.
SEND AND MAIL SAML FROM:reverse-path S: 250
F: 552, 451, 452
E: 500, 501, 502, 421
Deliver mail to one or more terminals or mailboxes. The argument field contains a reverse-path.
RESET RSET S: 250
E: 500, 501, 504, 421
Abort the mail transaction.
VERIFY VRFY string S: 250, 251
F: 550, 551, 553
E: 500, 501, 502, 504, 421
Verify that string identifies a user. If it is a user name, the full name of the user (if known) and the fully specified mailbox are returned.
EXPAND EXPN string S: 250
F: 550
E: 500, 501, 502, 504, 421
Expand the mailing list. The full name of the users (if known) and the fully specified mailboxes are returned, one to a line, in a multiline reply.
HELP HELP[ string] S: 211, 214
E: 500, 501, 502, 504, 421
Provide server information. The command may take an argument (e.g., any command name) and return more specific information as a response. .
NOOP NOOP S: 250
E: 500, 421
The receiver should send an OK reply.
QUIT QUIT S: 221
E: 500
The receiver sends an OK reply, and then closes the transmission channel. .
TURN TURN S: 250
F: 502
E: 500, 503
Reverse the roles of the two programs communicating over the transmission channel.