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That's pretty cool.

But there could be a reverse CAPTCHA where spammers send images with text that OCR engines can't recognize.

Any public breakthrough in OCR to solve the spam-CAPTCHA would actually hurt efforts to stop spammers who could beat CAPTCHAs to signup for more accounts. How ironic :)

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Spammers have employed this technique since roughly about the time they started sending images. See here for some examples:

http://www.jgc.org/tsc.html

Also, real OCR is too expensive to do at the volumes of mail being sent today. Gmail is likely using an OCR-esque technique that has lower precision but is much faster to process a given image.

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