Pay-per-email and the "Market Myth"
I like having the sender pay the reciever whatever the receiver demands in order to accept the email. The receiver would have costs for unknown addresses (maybe higher costs for large messages), maintain a white list of senders for which no or a reduced charge will be applied and would be able to cancel charges once an email is read, etc. The sender, once knowing the charge for accepting the email, could simply cancel the attempt to send or accept the charges. This would of course require a micropayment infrastructure but would allow for recipients to have a much larger say over what they receive and spend time dealing with.
There should also be much better integration between subscription lists and whitelists, so that when one subscribes to a mailing list, that mailing list address is automatically whitelisted.