Sender ID is an obsolete protocol derived from SPF developed by Microsoft, which validates one of the message's address header fields defined by RFC 2822. The algorithm aims to select the header field with the e-mail address "responsible" for sending the message. It is not the same as SPF or X-Sender-ID.
Sender ID (defined in RFC 4406) is a microsoft protocol derived from spf (hence the identical syntax), which 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 e-mail address "responsible" for sending the message.
Since 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.
SPF vs Sender ID
SPF and Sender ID are not the same. They differ in what they validate and what "layer" of the e-mail system they are concerned with. Sender ID is not the latest version of SPF – it is a obsolete and independent experiment. The "spf2.0" tag name is a historical accident. Neither is better because they address different problems. There is controversy because Sender ID is incompatible with existing specifications. Microsoft is aware of the problem and representatives of theirs have stated that they have no plans to fix it. There are practical work-arounds for SPF and Sender ID users.
