The TCP/IP Guide - SMTP Commands

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Table 252 lists the commands currently used in modern SMTP, in the order they are described in RFC 2821, with a brief description of each.

**Table 252: SMTP Commands**
Command Code	Command	Parameters	Description
*HELO*	Hello	The domain name of the sender.	The conventional instruction sent by an SMTP sender to an SMTP receiver to initiate the SMTP session.
*EHLO*	Extended Hello	The domain name of the sender.	Sent by an SMTP sender that supports SMTP extensions to greet an SMTP receiver and ask it to return a list of SMTP extensions the receiver supports. The domain name of the sender is supplied as a parameter.
*MAIL*	Initiate Mail Transaction	Must include a “FROM:” parameter specifying the originator of the message, and may contain other parameters as well.	Begins a mail transaction from the sender to the receiver.
*RCPT*	Recipient	Must include a “TO:” parameter specifying the recipient mailbox, and may also incorporate other optional parameters.	Specifies one recipient of the e-mail message being conveyed in the current transaction.
*DATA*	Mail Message Data	None	Tells the SMTP receiver that the SMTP sender is ready to transmit the e-mail message. The receiver normally replies with an intermediate “go ahead” message, and the sender then transmits the message one line at a time, indicating the end of the message by a single period on a line by itself.
*RSET*	Reset	None	Aborts a mail transaction in progress. This may be used if an error is received upon issuing a MAIL or RCPT command, if the SMTP sender cannot continue the transfer as a result.
*VRFY*	Verify	E-mail address of mailbox to be verified.	Asks the SMTP receiver to verify the validity of a mailbox.
*EXPN*	Expand	E-mail address of mailing list.	Requests that the SMTP server confirm that the address specifies a mailing list, and return a list of the addresses on the list.
*HELP*	Help	Optional command name.	Requests help information: general help if no parameter is supplied, otherwise information specific to the command code supplied.
*NOOP*	No Operation	None	Does nothing except for verifying communication with the SMTP receiver.
*QUIT*	Quit	None	Terminates the SMTP session.

Certain SMTP extensions make changes to the basic SMTP command set. For example, the AUTH extension specifies a new command (also called AUTH) that specifies an authentication method the SMTP client wants to use. Other extensions define new parameters for existing commands. For example, the SIZE extension defines a SIZE parameter that can be added to a MAIL command to tell the SMTP receiver the size of the message to be transferred.