Mailpile needs Hashcash support for fighting spam. See http://www.hashcash.org/.
Not a bad idea, we will look into this in more detail when we start focusing on spam.
An update: This isn't currently a priority; not a lot of mail providers support hashcash because it's ineffective aganst botnets. However apparently some do, and SpamAssassin does according to a brief Twitter conversation. So this sounds like a "nice-to-have", I would probably accept pull requests for it as long as they don't impact performance too badly.
Also related and interesting: guided-tour puzzle-based proofs of work: https://en.wikipedia.org/wiki/Guided_tour_puzzle_protocol
Hashcash support was added to SpamAssassin in 3.0. Docs detailing its use can be found at the Apache website. SpamAssassin, when configured, will reward mail with valid Hashcash tokens in the mail headers. The higher the cost, the better the reward.
Of course, calculating a valid token client side is CPU expensive. If the user is allowed to adjust the cost, it will take exponentially longer to find a valid token. However, this could be threaded to start working whenever a To: or Cc: address is entered, to minimize delaying sending the mail.
It should be mentioned that Microsoft implemented a client-side puzzle called "Postmark" and "Antispam tokens" in Outlook and Exchange to also help combat spam.
Hashcash has not seen wide adoption, so no doubt it should be a minor priority. If I can find the time, I'll submit a pull request.