[LeifCarrotson]

> The only parties who need a copy of an email are the sender and the recipient.

Emphasis on "need". Most recipients also like to have a third-party anti-spam service also have a copy of their email.

Assuming the encryption costs are low enough, spammers and virus senders would like nothing better than to cripple anti-spam learning tools by having each recipient recieve a cryptographically unique opaque blob. This would force users to develop their own training corpus and react to new spam and virus outbreaks individually.

You may argue that you could still use anti-spam locally, but it wouldn't be as good. While I wouldn't mind sending decrypted spam out to a server and getting updates to my local anti-spam program, no one would want to send legitimate mail, so the service would have no "ham" to train against.

I suppose encryption could help in the fight against spam by requiring CPU time to encrypt the email. 10 seconds per email would be hard to notice for a real human responding to messages, but might make spam unprofitable.

[NoGravitas]

I wonder if it would be good enough to use a public spam corpus built on volunteer contributions, while using only a private ham corpus.

[vectorjohn]

Mmmmmm, ham corpus :)