Honest - I can't tell if you are aghast that I would be suggesting that these email's would also have to pay to be received, or if, on quite the other hand, you are thinking that these sorts of messages would bypass what I envisioned, and am amazed that I would have missed something so obvious.
Just to be clear -- despite my not knowing to what point I am responding -- I envision a time when most people only receive email that someone, usually the sender, paid to have delivered.
For bonus points, an ISP could offer an automatic "pay by receiver" so that I could request that any email from say specified senders always be delivered to me, even if I have to pay the postage due.
Capitalism has worked every time it has been tried; and if we had micropayments working well, then capitalism could be applied to internet transactions that can't be subjected to that discipline currently, because their natural costs are a few to several orders of magnitude lower than can be efficiently paid with todays payment technologies.