Spam filtering is an exercise in trade-offs. There are many different technological approaches to the problem, and some are implemented better than others. But the different approaches also have their own inherent problems, and your tolerance of those problems is part of the equation in deciding whether the approach is a good one.
One of the more interesting approaches to spam filtering is challenge-response. The idea is that other people need permission to send you email and the product maintains a whitelist of such people. When someone not on the list sends you mail, the mail is held (at least for a while) and the sender is sent a message asking them to fill out a form, answer a question, and so on. In some cases, the user need only fill out the form, thereby proving they are not a bot sending out a zillion solicitations for increasing the size of certain body parts. In other cases, the email recipient may actually approve or decline specific lists of users.
When I reviewed a bunch of personal antispam tools for PC Magazine many months ago we only ended up reviewing one challenge-response product, Matador from MailFrontier Inc. Its a cute implementation and theres a lot to like technically about it, but too many challenges were sent out blindly to mailing lists and other bots and to people with whom we wished not to communicate.
And once again, just the other day, challenge-response got a shot in the arm when the Washington Post reported that Earthlink will offer it to their subscribers. Earthlink is already a leader in blocking spam. We had a high opinion of their filters, based on BrightMail. But as the Earthlink VP quoted in the story says, its really hard to write a good filter and hard to keep it working well.
The Post story also quotes other vendors in the challenge-response field, including an executive of MailBlocks, Inc., about the concept. That same day, MailBlocks put up a press release annoucing that they were suing Earthlink for patent infringement. No news is good news I guess.
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