They're into some mail server authentication thing.

Caller ID also relies on administrators adding lists of published e-mail servers to the DNS record for their Internet domains. Whereas SPF uses its own syntax for listing the domain addresses, Microsoft's Caller ID uses XML (Extensible Markup Language) to describe the valid e-mail servers, Levine said.

I don't really follow all the jargon, but it appears to me that this is an attempt to stop spam at the server level. The good news is that it spares individual users the pain of implementation (although non-spammers who routinely send their email through relays may be screwed). The bad news is that it adds a hack (or multiple hacks, if each of several similar proposals is adopted) to what used to be a simple, neat email protocol.

I still think that if Bill would just ship Outlook and Outlook Express with a default to only read text email, the spam problem would go way down.