Mr Gates' plans for anti-spam software include the inclusion of "human challenges" which force the e-mail sender to solve a puzzle or the computer sending the message to do a simple computation. This would be easy for a machine sending a few e-mails, but expensive and difficult when dealing with lots of spam.
The ultimate solution would be to make senders of e-mail pay a fee if their mail was rejected as spam. "Payment at risk", the electronic equivalent of a stamp, would not deter genuine e-mailers who would be confident their mail would be accepted, Mr Gates said.
Oh, that's brilliant. We get to solve puzzles every time we send an email! I predict computer game sales will plunge as avid gamers simply take to emailing themselves when bored. Take that X-box! :P
Article here: http://news.independent.co.uk/digital/news/story.jsp?story=484787
Taken from here: http://blog.fastcompany.com/archives/2004/01/25/the_end_of_spam.html