Message submission agent
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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]
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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.