"Yahoo! Research's CentMail resurrects an old idea: that levying a charge on every email sent would instantly make spamming uneconomic. But because the cent paid for an accredited "stamp" to appear on each email goes to charity, CentMail's inventors think it will be more successful than previous approaches to make email cost. They think the cost to users is offset by the good feeling of giving to charity."
"Some previous schemes, such as Goodmail, simply pocketed the charge for the virtual stamps. Another deterred spammers by forcing computers to do extra work per email; and Microsoft's version requires senders to decipher distorted text."
Here's an earlier post.Here's another story: Will Users Donate a Penny Per Email to Fight Spam, Yahoo Wonders, which notes
"It’s not clear how much the proposal would help, however, since so much of the spam is now sent using botnets, which are networks of zombie PCs whose owners have no idea their computers are part of a massive spamming organization."