Which of the following commands is used to specify the sender's email address in SMTP?

The command used to specify the sender's email address in SMTP is MAIL FROM. This command is crucial in the Simple Mail Transfer Protocol as it initiates the envelope sender's address that will appear in the "From" field of the email message. When a client sends an email, it first establishes a connection with the SMTP server, and the MAIL FROM command allows it to identify who the email is coming from. This helps the server handle the email appropriately, including enforcing any spam filtering or sender validation processes.

In contrast, the other commands serve different purposes in the SMTP protocol. RCPT TO indicates the recipient of the email, DATA is used to send the actual message content following the address commands, and HELO is used for the client to identify itself to the server. Thus, understanding the specific roles of these commands within SMTP is essential for effective email communication.