Challenges
From ASRG
| 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:
