What is the purpose of the SMTP command NOOP?

The purpose of the SMTP command NOOP is to check whether the connection to the server is still active. When a client sends a NOOP command, it essentially asks the server to acknowledge that the connection is still open and functioning. The server typically responds with a confirmation message, indicating that it is ready to accept further commands. This command is particularly useful in maintaining long-lived connections, as it helps to prevent timeouts that could result from inactivity.

While other commands will initiate email sending or close sessions, NOOP is specifically designed to ensure that the communication link is still operational without performing any other actions.