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