David Friedman proposes a solution to spam in chapter 6 of Future Imperfect. His approach combines a type of private currency market (which tickles the economist in me) with the powerful tool of asymmetric key cryptography (a massively underutilized technology, if ever there was one):
There is a simple solution to this problem – so simple that I am surprised it is not yet in common use. The solution is to put a price on your mailbox. Give your email program a list of the people you wish to receive mail from. Mail from anyone not on the list is returned with a note explaining that you charge five cents to read mail from strangers – and the URL of the stamp machine. Five cents is a trivial cost to anyone with something to say that you are likely to want to read, but five cents times ten million recipients is quite a substantial cost to someone sending out bulk email on the chance that one recipient in ten thousand may respond.
The stamp machine is located on a web page. The stamps are digital cash. Pay $10 from your credit card and you get in exchange 200 five-cent stamps – each a morsel of encrypted information that you can transfer to someone else who can in turn transfer it.
A virtual stamp, unlike a real stamp, can be reused; it is paying not for the cost of transmitting my mail but for my time and trouble reading it, so the payment goes to me, not the post office. I can use it the next time I want to send a message to a stranger. If lots of strangers choose to send me messages, I can accumulate a surplus of stamps to be eventually changed back into cash.
How much I charge is up to me. If I hate reading messages from strangers, I can make the price $1, or $10, or $100 – and get very few of them. If I enjoy junk email, I can set a low price. Once such a system is established, the same people who presently create and rent out the mailing lists used to send spam will add another service – a database keeping track of what each potential target charges to receive it.
What is in it for the stamp machine – why would someone maintain such a system? Part of the answer is seigniorage – the profit from coining money. After selling a hundred million five-cent stamps, you have five million dollars of money. If your stamps are popular, many of them may stay in circulation for a long time – leaving the money that bought them in your bank account accumulating interest.