port587

Port 587 is the Internet Assigned Numbers Authority (IANA) registered network port used for the submission of outgoing mail via the Simple Mail Transfer Protocol (SMTP). It is intended for client-to-server mail submission rather than server-to-server relaying on port 25. When used for submission, connections typically require authentication (SMTP AUTH) and use TLS to protect the message in transit, commonly via STARTTLS. This means a client connects to the mail submission server and, after starting TLS, authenticates to submit messages.

Many mail servers and providers require encryption and authentication on port 587. In addition, many Internet service providers block outbound connections to port 25 from end-user devices; port 587 provides a reliable alternative for legitimate mail submission. Historically, port 465 was used for SMTPS with implicit TLS; its status has varied over time, but modern practice generally favors 587 with STARTTLS for submission.

Usage considerations: clients connect to the mail submission agent over 587 to deliver messages to the mail transfer agent for onward delivery to recipients. Port 587 is not typically used for inbound submission or server-to-server relay in normal operation, though some servers may listen on 587 for legacy reasons. The standardization and widespread support make 587 the default submission port in many modern mail setups.