I'm not a fan of authentication as an anti-spam technique (see my
Inside RISKS column for details). That said, if you're going to use
the concept there are good and bad ways to do it. SPF (and hence
Microsoft's scheme) are really lousy ways to do it, for the reasons
John gave. Beyond that, a lot of people at the IETF had the
impression, rightly or wrongly, that Microsoft was trying to use its
patents as another weapon to use against open source software.
The IETF isn't nearly as nimble as it should be, but rushing to adopt a
bad solution is not a good idea.
--Steven M. Bellovin, http://www.cs.columbia.edu/~smb