ISPs would charge their customers a small amount for each email and would charge each other for receiving and delivering email. If a customer sends a large number of messages his ISP will realize that it's going to cost them a fortune if they send them to other ISPs for delivery and insist that the customer pay in advance. If I got a large email bill from my ISP I'd know that I was hacked and not pay it. They'd then trash the email and I'd do a virus scan.

If an ISP got a large number of messages from another ISP they'd suspect spamming and insist on payment in advance. If the sending ISP didn't pay then the email would be trashed.

If two ISPs decided to trade spamming rights then their customers would start getting spam again and would switch to a more reputable ISPs. The spamming ISPs would lose customers and their spamming operation would lose income.

It seems unlikely that all ISPs would make spamming deals. The big guys have too much to lose if their reputations are damaged. There might still be some spam but it would be greatly reduced.

If the above scheme isn't good enough then I have another idea. Each individual email user could set a price that he wants to be paid for receiving each piece of email. It need not be a large amount and each user could have a buddy list of people who would not be charged. Each sender of an email would include the maximum amount that he's willing to pay to have it delivered. If he offers at least what the recipient asks for then the email would be delivered and the sender would pay the recipient's fee. If the offer isn't enough then the email would be returned and the sender would be told how much he must offer to get it through. If it gets delivered, the sender's ISP would pay the recipient's ISP and the ISPs would settle up with their own customers monthly.

Since the recipients must be paid, the ISPs could not make profitable spamming deals and they'd have to put credit limits on their customers. If my computer were hacked by a spammer I'd soon reach my limit and the spam would stop. As in the previous idea, if an ISP received an unusually large amount of mail from another ISP they'd suspect spamming and demand payment in advance. They'd have to because they'd have pay their customers if they delivered it.