Now this looks very interesting – DomainKeys is a method for verifying email. It uses a public/private key infrastructure to verify the domain an email was sent from. Public keys are distributed by DNS.
Yahoo! has been granted a patent on the framework which annoys some but their license terms seems rather liberal, even allowing one to “yodel” it if that’s how your smtp works!
More: Jeremy, Simon, and Justin.
Several excellent comments on Jeremy’s site, including a link to Sender Policy Framework, a very similar technique already in use by the likes of AOL, Altavista and others. (I knew I’d seen idea this before!)