There's a growing and dangerous movement to try to stop spam by forcing all mail senders to provide ID with each mail they send. Signing mail is not a bad idea, in fact it's quite useful, but to stop spam you have to make everybody sign their mail.
In the past this was a non-starter because this means forcing everybody who mails you to get new mail sending software, or at least to have their ISP do this. But spam has made us so angry people are talking about doing this, even though we don't demand ID for paper mail that, in theory, can contain white powder that can kill you.
This would mean the end to anonymous mail and a lot more complexity in our mail systems. So I sat down and said, if you are ready to force people to get new software, could you stop spam with something more distributed and still allow anonymous mail.
Indeed you could, and I have a proposal outlined to combine CPU stamps, challenge/response and signature to end spam