Sender ID Framework and Intellectual Property Overview and FAQ
One of the claims people invoke for not deploying Sender ID is that it's a proprietary technology. Remember that Sender ID is a protocol jointly developed by Microsoft and Pobox.com.
To clarify some of these doubts, Microsoft just made available a FAQ regarding the need and use of Microsoft's royalty-free license for implementation of the Sender ID Framework. Here's a small excerpt from the Sender ID Framework and Intellectual Property Overview and FAQ:
Q1. Do end users need to sign this license?
A. No. End users do not need to sign this license.
Q2: Why is Microsoft asking people to take a license?
A: Microsoft is not aware of any issued patent claims covering the Sender ID specification, and Microsoft does not require anyone to sign a license with Microsoft to implement or use the Sender ID specification.