Sender ID is the email validation system designed to prevent spoofing. Sender ID, for the large part, relies on SPF with only slight differences.
The main difference between Sender ID and SPF is that Sender ID is there to improve the main deficiencies of SPF. The principal deficiency of SPF is that it does not identify the address in the header, which indicates the sending party. It often happens that the address checked by SPF is different from the one in the header of the email. The header addresses are shown to the users and serve to reply to an email. It means that SPF checks only the "mail from" email addresses.
Sender ID and SPF are almost identical, with only slight differences.
Microsoft held the key part of the patents of the Subscriber ID, which were licensed in a manner that did not conform to the rules of GNU General Public License, and which were not suitable for the implementation of free software. In 2006, Microsoft placed the patents under Open Specification Promise, which conforms to free and open source licenses.