> However, creating new publick/private key pairs is an incredibly > expensive operation, and one that a legitimate email wouldn't have to > do very often, but a spammer would if we just keep blacklisting their > keys. Uh? Creating a Diffie-Hellman public/private key pair is actually quite simple. Even an RSA pair is not all that hard, considering that a set of N prime numbers can generate N.(N-1)/2 key pairs. The logical consequence of authenticated e-mail is bound to be authenticated spam... -- Christian Huitema