"retired" technology. Notes References External links SpamHelp Challenge/Response Services A listing of challenge/response filtering service providers When Spam Filters Aren't Enough, Walt Mossberg of Wall Street Journal, March 22, 2007 Why Challenge-Response is a Bad Idea July 2006 Challenge-Response systems make matters worse February 2006 Challenge-Response Anti-Spam Systems Considered Harmful December 29, 2003 John Levine: Challenge-response systems are as harmful as spam May 2003 A Challenging Response to Challenge-Response May 2003 What You Need to Know About Challenge – Response Spam Filters 2003 Anti-spam Spam filtering Spamming