Simple Mail Transfer Protocol

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Simple Mail Transfer Protocol

[¦sim·pəl ′māl ‚tranz·fər ‚prōd·ə‚kȯl]
(computer science)
An Internet standard for sending e-mail messages. Abbreviated SMTP.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

Simple Mail Transfer Protocol

(SMTP) A protocol defined in STD 10, RFC 821, used to transfer electronic mail between computers, usually over Ethernet. It is a server to server protocol, so other protocols are used to access the messages. The SMTP dialog usually happens in the background under the control of the message transfer agent, e.g. sendmail but it is possible to interact with an SMTP server using telnet to connect to the normal SMTP port, 25. E.g.

telnet 25

You should normally start by identifying the local host:


You can then issue commands to verify an address or expand an alias:

VRFY VRFY postmaster

or expand a mailing list:

EXPN c-help

You can even send a message:

MAIL From:<> RCPT To:<> DATA What is the point? . QUIT

This is useful if you want to find out exactly what is happening to your message at a certain point.

See also Post Office Protocol, RFC 822, sendmail.
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(1) A similar-sounding acronym. See SNMP.

(2) (Simple Mail Transfer Protocol) The standard email protocol on the Internet and part of the TCP/IP protocol suite. SMTP defines the format of an email message and the message transfer agent (MTA), which is a mail server that forwards the mail. The SMTP server temporarily stores the messages until they can be relayed to another server or to a message store that uses the POP3 or IMAP4 access protocol to communicate with the user's email program.

Originally Only ASCII Text
SMTP was originally designed for only plain text, but MIME and other encoding methods enable executable programs and multimedia files to be attached to and transported with the email message. See TCP/IP, POP3, IMAP4, MIME and messaging system.
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