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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
      9  TCP/IP Key Applications and Application Protocols
           9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)
                9  TCP/IP Electronic Mail System: Concepts and Protocols (RFC 822, MIME, SMTP, POP3, IMAP)
                     9  TCP/IP Electronic Mail Delivery Protocol: The Simple Mail Transfer Protocol (SMTP)

Previous Topic/Section
SMTP Security Issues
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Pages in Current Topic/Section
Next Page
SMTP Replies and Reply Codes
Next Topic/Section

SMTP Commands
(Page 2 of 3)

Current SMTP Commands

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






The domain name of the sender.

The conventional instruction sent by an SMTP sender to an SMTP receiver to initiate the SMTP session.


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.


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.



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.


Mail Message Data


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.




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.



E-mail address of mailbox to be verified.

Asks the SMTP receiver to verify the validity of a mailbox.



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.



Optional command name.

Requests help information: general help if no parameter is supplied, otherwise information specific to the command code supplied.


No Operation


Does nothing except for verifying communication with the SMTP receiver.




Terminates the SMTP session.

Like FTP commands, SMTP commands are not case sensitive.

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.

Key Concept: The SMTP sender performs operations using a set of SMTP commands. Each command is identifies using a four-letter code. Since SMTP only supports a limited number of functions, it has a small command set.

Previous Topic/Section
SMTP Security Issues
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Pages in Current Topic/Section
Next Page
SMTP Replies and Reply Codes
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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