Sender ID rated incompatible to Open Source

Microsoft's Sender ID solution to spam looks intriguing, and until recently, it seemed destined to become an Internet standard. In brief, Sender ID is designed to ensure that e-mail originates from the Internet domain it claims to come from.
Indeed, if the framework is looking less appealing right now, it's only because some say that Microsoft's licensing terms are incompatible with Open Source. Today the Apache Software Foundation has rejected Sender ID, arguing that while the license is royalty free, it is incompatible with Open Source, and thus the foundational properties of the Internet.
The current Microsoft Royalty-Free Sender ID Patent License Agreement terms are a barrier to any ASF project which wants to implement Sender ID. We believe the current license is generally incompatible with open source, contrary to the practice of open Internet standards, and specifically incompatible with the Apache License 2.0. Therefore, we will not implement or deploy Sender ID under the current license terms.
Additionally, Microsoft claims to have patents relating to Sender ID, but those patents have not been disclosed—something which makes many people rather nervous.