Message submission
The step where a mail program or sending tool hands a new message to its own provider's server, defined by RFC 6409 as a service separate from relay between servers.
What it means
RFC 6409 splits sending in two. Submission is the user's program talking to a Mail Submission Agent on port 587 (or 465 with implicit TLS, RFC 8314), where the user authenticates and the server may complete the message: it may add a missing Date and should add a Message-ID if there is none. Relay is that server passing the message on to the recipient's MX on port 25.
The split lets providers require authentication and enforce sending limits at submission while port 25 stays for traffic between servers. RFC 5068 notes that some access providers block outbound port 25, and requires that they never block port 587. When a sending tool asks for SMTP settings, it wants the submission server: host, port 587 or 465, username and password.
Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
All 432 terms in the glossaryThe wiki: blocklists and bounce codes