There's been a lot of talk lately about fighting spam, but I've been wondering lately about the problem of e-mail viruses.
At work we're getting a steady 200-300 Klez-infected e-mails a week, and it's been that way ever since it first hit. It seems like it's never going to stop - people just don't ever get a clue. And that's just one particular virus - as soon as a new and better one comes around, people will fall for it all over again.
Maybe the thing to do is to start fighting e-mail viruses the way you fight spam - by identifying "virus relays" and blacklisting them. Eventually ISPs would have to implement virus scanning on their SMTP servers, and would push the problem back to the edges of the network where the actual infected machines would have to be cleaned up and better-protected if they wanted to ever send any e-mail.
I know that's pretty impractical, but I suppose you could still try identifying "virus relays" and periodically send them reports of how many infected emails were received from their system, and suggest they do something about it rather that just blindly passing the problem along.