Challenges

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Anti-spam technique: Challenges
Date of first use: 1990s
Effectiveness: Medium
Popularity: Medium
Difficulty of implementation: Medium
Where implemented: MTA or MUA
Harm: High

Challenges or Challenge/Response (C/R) attempts to validate unknown senders by replying to their mail with a challenge message. If the sender responds to the challenge in a way the C/R system approves of, the mail is delivered and the sender is whitelisted.

C/R systems are widely considered to be abusive and ineffective. Problems include: