Message submission agent

A message submission agent (MSA), or mail submission agent, is a computer program or software agent that receives electronic mail messages from a mail user agent (MUA) and cooperates with a mail transfer agent (MTA) for delivery of the mail. It uses ESMTP, a variant of the Simple Mail Transfer Protocol (SMTP), as specified in RFC 6409.^[1]

Many MTAs perform the function of an MSA as well, but there are also programs that are specially designed as MSAs without full MTA functionality.^[2] Historically, in Internet mail, both MTA and MSA functions use port number 25, but the official port for MSAs is 587.^[1] The MTA accepts a user's incoming mail, while the MSA accepts a user's outgoing mail.

The computer running an **MSA** is also known as the *outgoing mail server*.

^ ^a ^b Gellens, R.; Klensin, J. (November 2011). "Submission Identification". Message Submission for Mail. IETF. sec. 3.1. doi:10.17487/RFC6409. STD 72. RFC 6409. Retrieved November 14, 2013.
^ Costales, Bryan; Assmann, Claus; Jansen, George; Shapiro, Gregory Neil (2007-10-26). sendmail: Build and Administer sendmail. "O'Reilly Media, Inc.". ISBN 978-0-596-55534-4.

[rfc6409-1] Gellens, R.; Klensin, J. (November 2011). "Submission Identification". Message Submission for Mail. IETF. sec. 3.1. doi:10.17487/RFC6409. STD 72. RFC 6409. Retrieved November 14, 2013.

[2] Costales, Bryan; Assmann, Claus; Jansen, George; Shapiro, Gregory Neil (2007-10-26). sendmail: Build and Administer sendmail. "O'Reilly Media, Inc.". ISBN 978-0-596-55534-4.

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