"In a recent message to the MARID list RMS weighs in on the licensing issues of Sender-ID/SPF and Microsoft: 'Microsoft's Sender-ID license is directly incompatible with free software regardless of which free software license is used. Free software means users are free to run it, study and modify the source, and to redistribute it with or without changes. Free to do so means there is no requirement to ask or tell anyone that you are doing so.'" "MARID" stands for MTA Authorization Records in DNS; here's the IETF MARID working group's charter.
Stallman's message continues: 'The Microsoft license for Sender-ID directly forbids release of software with all these freedoms, so it is impossible for any program to be free software under Microsoft's regime.
I've been expecting to see something like this ever since Gates started talking about spam. This license is an example of Microsoft's strategy for killing off free software as an alternative to Windows. Microsoft first patents something, then incorporates it into a format or protocol, then tries to make it de rigueur while excluding those it wishes to exclude. In the absence of resistance, Microsoft has a good chance of imposing whatever standards it likes. Let us, therefore, resist it here and now.'""