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Challenge/response spam filtering
Description: This «selfish» method of spam filtering replies to all email with a «challenge» — a message only a living person can (theoretically) respond to. There are several problems with this method which have been well known for many years.
— Does not scale: If everyone used this method, nobody would ever get any mail.
— Annoying: Many users refuse to reply to the challenge emails, don't know what they are or don't trust them.
— Ineffective: Because of confusion about these emails, many of them are confirmed by people who did not trigger them. This results in the original malicious email being delivered.
— Selfish: This is the problem we are mainly concerned with. By using challenge/response filtering, you are asking innumerable third parties to receive your challenge emails just so that a relatively few legitimate ones get through to the intended recipient.
SpamCop abandoned this method of filtering after a short test period in 2001.
Solution: Do not use challenge/response filtering. Although it may stop most unwanted email for the person shielded by it, it generates more unwanted email for others.
Since more and more sites will rightly block these challenge emails, you can never be sure they will reach their target even when they are not misdirected themselves. So these systems will lose legitimate mail in an attempt to stop unwanted mail.
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