I was just flagging a lot of spam and came up with an idea that could help solve this problem. Basically it involves having my mail server ask the supposed sender really sent that message. It's really a lot like Jabber's reverse domain look up, but on a per-message basis. Email (sendmail, qmail, and friends) really needs this.
Harder than you expect
See MARID, and all of the things (SenderID, etc) that came out of it.
The core problem is legitimate things that relay, like mailing lists. There's also more than one "from" address to deal with, which also complicates things.