What command is used to check if an SMTP server is operational?

The command used to check if an SMTP server is operational is NOOP. The NOOP command essentially serves as a "no operation" command, which prompts the server to acknowledge that it is ready and functioning without performing any specific task. When this command is sent to the SMTP server, a proper functioning server will respond with a positive acknowledgment, confirming its operational status.

Other commands like HELO, MAIL FROM, and DATA have distinct functions in the SMTP protocol. HELO is used to initiate a conversation with the server and identify the client. MAIL FROM is utilized to begin a mail transaction and specifies the sender's email address. The DATA command follows the MAIL FROM command to initiate the transfer of the actual message content. While these commands are essential in the SMTP communication process, they are not specifically intended to verify the server's operational state like the NOOP command, which serves this specific purpose effectively.