How about this take on e-mail postage. We know spammers/phishers send lots of e-mail, but receive very little or none. We use that to our advantage
Before sending e-mail, a sender buys postage, and it goes into their account. Maybe a penny a stamp give or take. When an e-mail is send, a stamp is taken out of the sender account and put into an escrow for each recipient. The e-mail is digitally signed for the escrow id, and sent like normal, but all spam filtering services then check the signature along the way.
When a recipient opens an e-mail, The escrow stamp set assigned for them is transfered to their account (e-mail client, or service provides this). Note: it can only be collected once for each person per e-mail, and it only goes to the account associated with the e-mail.
So after an initial stamp purchase, postage will transfer back and forth, and a normal user should never have to purchase postage again. A person, or company that sends lots of e-mail will have to keep buying postage to send. PHishing and spamming becomes economically difficult.
More reputable spammers/companies will have to buy postage to stay in business.
One last thing, users will be able to sell back stamps when there account starts to fill up, but at less of a price, to pay for the service and keep the validation servers running. So stamps are purchased at retail prices, and sold back at whole sale prices. Spammers/Hammers that stay in business end up paying for the service.
There is much more details, and ideas that can go along with this, but for the sake of brevity, I'll keep int at that.
"'Tis true, 'tis pity, and pity 'tis 'tis true." -- Poloniouius, in Willie the Shake's _Hamlet, Prince of Darkness_