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In-Boxes That Fight Back
But the spam-blocking technique that's attracted the most attention among start-ups recently is a very simple one: Challenge-response (CR) technology. When your mailbox is protected by a CR system, anyone who tries to contact you will be greeted with a response saying something like "click on this link to deliver this message" or "type in the word you see in the box above." Well-designed CR utilities won't challenge mail from known correspondents or mail that you specifically asked to receive.