A mail submission agent or MSA is a computer program or software agent which receives electronic mail messages from a mail user agent (MUA) and cooperates with a mail transfer agent (MTA) for delivery of the mail.
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Rationale
Many MTAs act as an MSA as well, but there are also programs that are specially designed as MSAs without full MTA functionality. Historically in Internet mail, both MTA (acceptance of locally-destined mail from other domains) and MSA (acceptance of submitted mail from local users) functions were both performed by MTAs using the same protocol (SMTP). Separation of the MTA and MSA function produces several benefits: One benefit is that an MSA, since it is interacting directly with the author's MUA, can correct minor errors in a message's format (such as a missing Date, Message-ID, To fields, or an address with a missing domain name) and/or immediately report an error to the author so that it can be corrected before it is sent to any of the recipients. An MTA accepting a message from another site cannot reliably make those kinds of corrections, and any error reports generated by such an MTA will reach the author (if at all) only after he has already sent the message. Another benefit is that separating the MTA and MSA functions makes it easier for an MTA to deny relaying, that is to refuse any mail that is not addressed to a recipient at a domain that is served locally. By contrast, an MSA must generally accept mail for any recipient on the Internet, though it only accepts such mail from authors who are authorized to use that MSA and who have established their identity to the MSA via authentication. In times when both mail submission and acceptance of incoming mail were usually accomplished using the same protocol and the same server, the ability to send mail to arbitrary destinations without authentication allowed spammers to use MTAs as a means of distributing spam (since a single message transaction can request that an MTA relay a message to a large number of recipients), and also made it more difficult to trace a message to its origin. Yet another benefit is that MSAs and MTAs can have different policies for filtering of spam. Most MSAs require authentication in the form of a username and password provided by the author. Any messages received by such an MSA are therefore traceable to an author who has a direct relationship with the MSA, and who can be held accountable for his actions. This allows the MSA to have either no spam filtering, or more permissive spam filtering than an MTA that exists for the purpose of accepting incoming email from other domains. It is difficult to establish trust in mail sent between arbitrary domains, because there is generally no direct relationship between those domains via which trust, or even identity, can be established. In the absence of such trust, an MTA must generally rely on heuristics and third-party reputation services to distinguish spam from legitimate traffic, and both of these mechanisms have a history of being error-prone. The separation of MSA and MTA therefore avoids the use of unreliable spam recognition mechanisms during mail submission, and increases the probability for legitimate mail to be delivered successfully.
Implementation
Many internet service providers and enterprise or institution networks restrict the ability to connect to remote MTAs on port 25 (the usual SMTP port). Availability of a Mail Submission Agent on port 587 enables nomadic users to continue to send mail via their preferred submission servers even from within others' networks. Using a specific submission server is a requirement when sender policies are enforced. Most of the benefits mentioned above may also apply to authenticated MTA, that is port 25 after the user logs in. In fact, the relevant server software is often the same for both services. However, the very fact that the service can be made available on a different port can be considered a further benefit, as it allows to circumvent restrictions on port 25.
See also
References
- C. Hutzler, D. Crocker, P. Resnick, E. Allman, T. Finch, Email Submission Operations: Access and Accountability Requirements, RFC 5068, November 2007.
- R. Gellens, J. Klensin, Message Submission for Mail, RFC 4409, April 2006.
- R. Gellens, J. Klensin, Message Submission, RFC 2476 (obsoleted by 4409), December 1998.
- J. Klensin, ed. Simple Mail Transfer Protocol. RFC 2821, April 2001.
- Selected MARID-Related Terms and Acronyms. Techmoids.org. Retrieved on February 24, 2005.
- SMTP is not secure. Kasoft Central. Retrieved on February 24, 2005.
MSA as an abbreviation can have other meanings: see the MSA disambiguation page.
