MSA
A Mail Submission Agent: the server that accepts new mail from a user's program, checks and completes it, and hands it to the mail system for delivery.
What it means
The MSA is the server behind a program's SMTP settings, on port 587 or 465. RFC 6409 lets it do what a relay may not: require authentication, check that the user may send as the address given, and complete the message, for example adding a Date or Message-ID header that is missing. RFC 5598 describes it as serving two interests at once, the author's in getting the message posted and the mail service's in enforcing its rules.
The MSA is also where providers apply per-account sending limits, which is why a single mailbox can be stopped from sending while its domain is in good standing; Gmail reports a reached daily sending limit with 550 5.4.5. Once accepted, the message passes to an MTA for relay.
Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 5598, section 4.3.1: Mail Submission Agent (MSA)rfc-editor.org
- RFC 6409, section 6.1: Enforce Submission Rightsrfc-editor.org
- Google Workspace Admin Help: Gmail SMTP errors and codesknowledge.workspace.google.com
All 432 terms in the glossaryThe wiki: blocklists and bounce codes