Which SMTP command is used to initiate the mail transfer?

The command used to initiate the mail transfer in SMTP is HELO or EHLO. When a client connects to an SMTP server, it first sends either of these commands to introduce itself. The HELO command is used for communicating with legacy servers, while EHLO extends the capabilities by allowing the client to identify itself using the Extended SMTP (ESMTP) protocol, which supports additional features like authentication and enhanced message handling.

This step is crucial because it establishes a session between the client and the server, enabling further communication and ensuring that both parties are prepared for the data exchange that follows. The use of HELO or EHLO sets the stage for the subsequent commands related to sending the email, such as specifying the sender and recipient addresses with MAIL FROM and RCPT TO, and finally sending the actual message content with the DATA command.

Ensuring a clear understanding of this initiation process is fundamental to grasping how email transfer works within the SMTP protocol.