Port 587 supports SMTP submission and submission (UDP reference coverage) across distinct transports. Review each protocol before writing firewall or monitoring policy.
Authenticated client email submission, commonly upgraded with STARTTLS.
User agents and applications sending messages through a mail provider.
Client to server
Require authentication and TLS; submission policy should be separate from server-to-server relay on port 25.
submission (UDP reference coverage) is documented as a network service convention for port 587 over UDP.
Use this reference entry when identifying or validating systems documented to use 587/UDP.
Varies
Confirm the listening process and current vendor documentation before permitting traffic; a listed convention does not prove the active service or its security posture.