Microsoft wants its Sender ID framework to be more popular to get back more revenue into its coffers and its willing to do that under its Open Specification Promise (OSP) program. Sender ID is an anti-spam technology born out of the Internet Engineering Task Force (IETF), which Microsoft participated in and holds a number of patents. Up until the OSP release, those patents made the Microsoft implementation of Sender ID incompatible with more relaxed, open licenses, such as the GNU General Public License.

According to Mac News :

However, intellectual property questions arose two years ago about possible patent claims Microsoft might be likely to make over the application. After that, the likelihood that Sender ID would provide the platform for an industrywide standard diminished amid fears by some developers that Microsoft would go after them for royalties.

“There have been lingering questions from some members of the development community about the licensing terms from Microsoft and how those terms may affect their ability to implement Sender ID,” said Brian Arbogast, corporate vice president of the Windows Live Platform Development Group at Microsoft. “By putting Sender ID under the Open Specification Promise, our goal is to put those questions to rest and advance interoperable efforts for online safety worldwide.”