Anti-Spam Technologies Review - Part 2

Challenge Response

The idea behind Challenge-Response (CR) systems is to accept email from only permitted senders. Email from a new sender is temporarily held without delivery. The new sender is sent an email that provides a challenge (usually a click on a URL or reply email). After completing the challenge, the new sender is added to the list of permitted senders and the original email is delivered. The belief is that spam senders using fake sender email addresses will never receive the challenge, and spam senders using real email addresses will not be able to reply to all of the challenges.

Pros: Challenge response, if well designed, is very effective against spam. Users can be sure that no spam will get through to their email box.

Cons: 1. If widely deployed, it can easily cause CR deadlock. For example, if A and B both use challenge response and it's the first time A sends an email to B, B's challenge system will send an challenge back to A, A's system send another challenge to B. Since neither user actually receives the challenge, neither user will receive the email.

2. It will stop many unsolicited mail as well. For example, mailing lists and automated systems are unable to respond to challenges. Therefore, they will suffer just as spammers do. And some unsolicited email which is sent by your acquaintance directly from a web site, will get blocked as well.

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