Yahoo! and Microsoft are giving serious thought to the idea of e-mail "postage" that costs senders a small fee, company officials said.
The admissions come in the wake of Microsoft founder Bill Gates' January comments in Davos, Switzerland suggesting the spam problem will be defeated by a number of different solutions, but "in the long run, the monetary method will be dominant." [InternetNews]
I've occasionally collected flak from people who love the Bayesian filtering approach for asserting that the monetary approach could be a workable solution. But I have thought and continue to think it makes sense. It's just a matter of getting a critical mass of businesses to implement it. Yahoo and Microsoft together qualify as critical mass, I believe.
Essentially the idea is to either charge for all email (in which case spammers couldn't afford it) or only charge for emails rejected as spam. This can be done with "real" money, or with a currency based on expenditures of CPU cycles.
There are a number of other feasible approaches than cost-based. The main thing is some solution or set of solutions reaching critical mass followed by broad adoption by the industry. Spam is going to be all but eliminated in the next couple of years, as that critical mass is reached.