Anytime anyone sends a mail to my server, I want to be paid 2 cents.
And then, no one will want to send _you_ email. Spam or otherwise.
You would also want to be able to accept mail from certain senders for free.
Which I guess is how you would avoid killing off legitimate mass mailing (like nanog).... And would that be set up sort of like peering? So instead of just major network's peering with other major networks, all of the sudden everyone running a mail server has to work out "peering" (clearly a different type, but I think you would see a lot of the same mess, with people setting their own requirements, etc) with everyone else who runs a mail server that you regularly get mail from. I gather that peering negotiations are difficult, even between the large networks - can you imagine what a mess this would be? ack! Of course, the flip side is that if I begin a business that runs a email service that won't charge to receive mail, then I might be operating at a competitive advantage for attracting business customers (generally an attractive demographic) who don't want people to have to pay in order to contact them. So you end up with either no isp willing to implement your system, or with them having to run parallel mail systems - one free, one fee.
What I envision is some sort of micropayment protocol extension to SNMP. something like you exchange helo's, mail from, and rcpt to's, and the receiving server says to the sender "That will be x cents please", at which point the server sends some sort of cert-signed digital cash.
A downside of this - if you're able to implement this, then it becomes trivial to impose some kind of an "email tax". While that would be unpopular, once you start charging people for email, adding on 1 more cent as tax, no big deal, right? etc. etc. etc. I think you'd quickly see taxes here and a lot of other places on the net, as a result. IMHO, that would be a bad outcome.
I'm not sure how you would bootstrap this or if it will ever be possible. I just think that if we could get even $0.02 per email from the spammers a lot of them would stop.
I think you'd be throwing the baby out with the bath water here. Yes you'd kill spam, but you'd kill a lot more, too. And I think you'd quickly find that the cost of administrating this system and dealing with the billing and agreements and disputes would take up as much or more time that is spent now on spam. Clearly the current system (blacklists, etc) for dealing with spam isn't perfect. But it is evolving - and if more jurisdictions in the US started putting laws on the books that made it easier to track down, and shut down spammers and people (isp's) that knowingly provide them service, that it will be possible to cut out most of the spam without going to drastic measures like this.