Port 25 supports SMTP and smtp (UDP reference coverage) across distinct transports. Review each protocol before writing firewall or monitoring policy.
Server-to-server email delivery and mail relay.
Mail transfer agents exchanging messages; authenticated client submission normally uses port 587.
Bidirectional
Prevent open relay, enforce anti-abuse controls and use STARTTLS where supported. Port 25 is often blocked for consumer outbound traffic.
smtp (UDP reference coverage) is documented as a network service convention for port 25 over UDP.
Use this reference entry when identifying or validating systems documented to use 25/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.