What does the SMTP command MAIL FROM specify?

The SMTP command MAIL FROM is specifically designed to specify the sender's email address. When a mail transfer agent (MTA) receives this command, it understands that the following address provided is the origin of the email message being sent. This command is a critical part of the email-sending process, as it establishes the sender's identity, which can be used for various purposes such as verifying the sender's authenticity, handling replies, and displaying the "From" address in the recipient's inbox.

In the context of SMTP protocol interactions, the sender's address is essential for routing and delivering the email correctly and for potential bounce-back messages if the delivery fails. Understanding this command is fundamental to comprehending how email systems process and relay messages.