Many sender verification anti-spam systems are available. The most commonly used sender verification system is probably challenge/response, where a recipient requires a sender (who is not already whitelisted) to reply to a challenge message in order to verify that the sender’s message is not spam. A variation on this is the sender compute method, where the recipient requires a sender (who is not whitelisted) to compute an algorithm to prove that he or she is not a bulk emailer.

We chose to cover Camram, which is a sender compute verification system, along with facilities for challenge/response and a very nice GUI for the CRM114 Bayesian filter. ASK is a traditional challenge/response system that implements verification via MD5 email ...

Get Slamming Spam: A Guide for System Administrators now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.