What command is used to indicate the start of a new mail session?

The command that indicates the start of a new mail session is HELO or EHLO. These commands are fundamental to the Simple Mail Transfer Protocol (SMTP) as they establish communication between the client and the mail server. When a client wants to send an email, it sends HELO or EHLO to identify itself to the server.

HELO is the original command used for the SMTP communication, while EHLO is an extended version that requests the server to provide a list of its capabilities, enabling the use of more advanced features. This introduction not only initiates the session but also establishes a set of expectations for both the client and server regarding the transaction that will follow.

This command contrasts with others like MAIL, which is used to specify the sender of the message, and DATA, which is used to send the actual content of the email, or QUIT, which ends the mail session. These commands function after the initial session is established, making HELO or EHLO essential as the first step in the SMTP process.