It looks like the big email server guys are working on different schemes which might help with spam. The deal is that your receiving email server might be able to figure out who really sent a particular email. You'd be able to set for yourself how to handle emails for known and unknown senders. For example, you might tell your emailer to only accept email from known domains… and then you can whitelist known senders of emails, and blacklist the rest. There would also be variations where you could let in email from unknown people… you choose.
This really deserves longer treatment… and I'll be doing more research.
I feel very uncomfortable with Microsoft leading or having a major role in this, given their record regarding security and how they treat customer. I fear that they'll implement this in MSN and Hotmail without serious testing.