Gates has also suggested requiring people to flat-out pay for each piece of email that they send

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I've personally had a lot of luck with spam filters, especially the one in the latest version of Eudora. Out of the 100+ spam messages I receive a day, only one or two make it into my in-box.
Spam is the unfortunate result of a free mail system. I think it would be best to just let things stay status-quo. The more you fight spam, the more the fundamental nature of email will change (there probably will be a very real push to charge postage for email in a few years, and people will use spam to help argue that policy).