Sender ID is a Microsoft protocol derived from SPF that validates one of the message's address header fields defined by RFC 2822. Which one it validates is selected according to an algorithm called PRA (Purported Responsible Address, RFC 4407). The algorithm aims to select the header field with the email address "responsible" for sending the message. Because it was derived from SPF, Sender ID can also validate the MAIL FROM, but it defines the new PRA identity to validate, and defines new sender policy record tags that specify whether a policy covers MAIL FROM (called MFROM by Sender ID), PRA, or both.