Earlier, Microsoft's submission of the SenderID specification (anti-spam) to the IETF was rejected by a lot of companies and this sent Microsoft back to the drawing board. This was due to concerns about the spec falling in certain patented areas of Microsoft's world. Now the SenderID spec has been revised to take care of these concerns and now Microsoft has AOL on their side.
The idea of the spec is that mail servers will match up a domain name from the email address to make sure that the email really came from that location. If it did not, then SPLAT ... the email is gone! Read more about it here.