What port does the Simple Mail Transfer Protocol (SMTP) primarily operate on?

The Simple Mail Transfer Protocol (SMTP) primarily operates on port 25. This protocol is used for sending emails from a client to a server or between servers, making it a fundamental component of how email functions over the Internet.

Port 25 is designated for SMTP traffic, which allows for the transmission of email messages. It was originally defined in the early days of email communications and remains the standard port for SMTP, even as additional ports, like 587, are recommended for secure submissions, especially for authenticated email sending.

Understanding the specific port associated with SMTP is crucial for IT professionals, as it helps in configuring email servers, developing firewall rules, and troubleshooting email delivery issues. Knowledge of these ports ensures that email communication can proceed without being blocked due to misconfigurations.