Just thought it would be a funny idea... but if there was an email system that used bitcoin (small fees) for sending messages. You could make it so that if someone sent you a message and it cost them a small fee... you could somehow reply and return the fee to the sender. this would make it even cheaper (since no one will probably respond to spam but they might to a valid user)
I suppose it would work like this: There is some kind of bitcoin clearing house where you set up an account. You can input a bitcoin amount and it will allow you to send a message. When the message is sent the bitcoin fee is put somewhere in reserve. If the person you email replies... then the fee is returned to you. Otherwise it's used to finance the actual service itself. Whatever the fee amount would be something low but not so low that a spammer could afford to spam the network .
Good idea, why not just attach (multisig?) transaction to the email, and if receiver wants to redeem it, he can sign it and broadcast it. So that sender and receiver don't need trusted third party to confirm the payment, but also bitcoins appended to the message can't be stolen by intermediary. Also if the message isn't delivered, the transaction expires and payment returned to sender.
Edit: But, why can't everyone just use PGP? Encrypting/signing millions of emails is computationally expensive, mass gathering of pgp fingerprints is nontrivial. That would lower spam volume, too.
