Challenge-response
From ASSPSMTP
A Challenge-response (or C/R) for anti-spam purposes is a mechanism that, when a sender is unknown to the spam filter, will respond to an e-mail with a challenge response e-mail back to the sender; requesting that a real-person verification take place. This verification typically consists of a cryptographically-enhanced (tagged) confirmation URL that may contain a confirmation code, expiration time, or both; and may also include a reverse Turing test such as CAPTCHA.
- For more information see the Challenge-response (C/R) spam filtering at Wikipedia.
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