How do you specify the sender's email address in SMTP?

In the Simple Mail Transfer Protocol (SMTP), specifying the sender's email address is done using the MAIL FROM command. This command is part of the SMTP protocol's initial connection phase where the sender's information is communicated to the server.

When a mail client connects to an SMTP server to send an email, it first uses the HELO (or EHLO) command to introduce itself. Following this, the client employs the MAIL FROM command to indicate the sender's email address. This is essential for the SMTP session, as it helps the server understand who is sending the email, allowing for proper handling and delivery.

In contrast, other commands serve different purposes: the RCPT TO command is used to specify the recipient's email address, the DATA command indicates that the email message content will follow, and the HELO command is for identification purposes. Therefore, the MAIL FROM command is specifically designated for declaring the sender’s address, making it the correct choice.