I would like to know some information filtering systems?

Challenge/response or C/R systems are a type of spam filter that responds to incoming e-mail messages by sending a challenge to the claimed sender of the e-mail. The sender may then perform some action outlined in the challenge message to assure delivery of their message, which otherwise will not be delivered or may be delivered with a lower priority. These strategies are currently controversial among e-mail programmers and system administrators since spammers often forge the e-mail addresses of the claimed sender and so these challenges often go to the wrong people. Even when the challenge is sent to the real sender, they often dislike the challenges on mail they feel should have been easily classified as non-spam and delivered.