Which port is used for SMTP submission (mail submission service)?

• A. 25 TCP
• B. 587 TCP
• C. 2525 TCP
• D. 465 TCP

Answer: (B)

The key idea is that SMTP submission, where a mail client hands a message to a server, uses a dedicated port designed for client-to-server submission with authentication. That port is 587. It’s defined for mail submission and is the standard path that mail clients use to submit messages to a mail server, typically with TLS (STARTTLS) and authentication.

Port 25 is the traditional port for MTA-to-MTA delivery, not for user submission, and is commonly restricted for direct client use to prevent abuse. Port 465 was once used for SMTPS (SMTP over SSL) but isn’t the current standard for submission; modern practice is to use 587 with STARTTLS. Port 2525 is an alternative, nonstandard port that some providers use, but it’s not the official submission port.