Message submission gives clients and applications a service with policy appropriate for new outgoing mail, separate from MTA-to-MTA relay.
Port 587 is reserved for message submission. Port 465 is registered for submissions using implicit TLS.
A submission service can authenticate users, enforce sender policy, add missing message fields and reject malformed submissions before the message enters normal relay.
TLS
RFC 8314 recommends TLS for submission/access and prefers implicit TLS where practical, while also describing STARTTLS submission during the transition.