This was inevitable - email "stamps" that will let senders bypass spam filters. AOL and Yahoo are both doing it:

The Internet companies say that this will help them identify legitimate mail and cut down on junk e-mail, identity-theft scams and other scourges that plague users of their services. The two companies also stand to earn millions of dollars a year from the system if it is widely adopted.

AOL and Yahoo will still accept e-mail from senders who have not paid, but the paid messages will be given special treatment. On AOL, for example, they will go straight to users' main mailboxes, and will not have to pass the gantlet of spam filters that could divert them to a special bulk e-mail box or strip them of images and Web links.

There are a couple of downsides to this approach though. First, the initial stages of this will likely see multiple "pay to send" implementations, which will be a pain to deal with. That won't be easy to solve, either - email (unlike postal mail) isn't run by governments, so getting a single, unified system that works across borders won't be easy.

The second problem is bigger, IMHO. These fees guarantee that mail will bypass the filters of the provider (Yahoo, AOL) - but not that the email will bypass any client filters in place. You can get a Yahoo address and have it forwarded to another personal account, or use POP to get your mail downloaded outside a browser. At which point, a client side filter could ensnare the email. For example, mail sent to my cincom address has to first get past the corporate filter, and then past the client side filter on my end. I've had mail get snagged by one and not the other, as well as by both.

The upshot: the fee you pay does not actually guarantee delivery. Which is going to be a problem.