Require a small fee to send an email, e.g. $0.01. Small enough not to matter for legitimate users, large enough to make spam unprofitable.
Bitcoin or some other cryptocurrency would be ideal for facilitating micro-transactions like this. Interestingly, the Hashcash concept was originally designed to fight spam, and later became one of the important ideas that made the invention of Bitcoin possible: http://en.wikipedia.org/wiki/Hashcash
Bitcoin or some other cryptocurrency would be ideal for facilitating micro-transactions like this. Interestingly, the Hashcash concept was originally designed to fight spam, and later became one of the important ideas that made the invention of Bitcoin possible: http://en.wikipedia.org/wiki/Hashcash