I thing SPAM is more a political problem than a technical one !
A lot of smart technical solutions exist for a long time now but SPAM remains a problem.
Main mail service providers like Google and Hotmail could choose and impose new rules in mail exchange to the community, by rejecting mails from servers not following them. Then I thing SPAM would be reduced to the minimum.
My solution below is deliberately superficial because a real solution would require long debate of mail gurus to avoid any weakness. I'm only one and maybe not a guru :-)