Message submission agent

One benefit is that an MSA, since it is interacting directly with the author's MUA, can correct minor errors in a message 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.

The accessibility of an MSA on port 587[3] enables nomadic users (for example, those working on a laptop) to continue to send mail via their preferred submission servers even from within others' networks.

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.

Domain example.com can publish its record like so:[6] RFC 6409 requires that clients are authorized and authenticated to use the mail submission service, e.g., as described in SMTP-AUTH (ESMTPA), or by other means such as RADIUS, public key certificates, or (the mostly obsolete) POP before SMTP.

The MSA must check that the submitted mail is syntactically valid and conforms to the relevant site policies.