Challenge-Response Spam Blocking Challenges Patience
In a way, it's the perfect spam blocking technique. But Security Supersite Editor Larry Seltzer has some concerns about challenge-response spam blocking. And you'll never know how badly it's really working.
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.
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