http://email.about.com/od/outlookspamfilters/Spam_Fighting_Tools_for_Outlook.htm

Outlook comes with a spam filter, but it is good, not perfect. There are many anti-spam plug-ins that claim they can do better, and some of them really can. Find them here.

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http://www.appriver.com/services/spam-and-virus-protection/index1.aspx

http://www.chacocanyon.com/resources/email-challenges.shtml

http://en.citizendium.org/wiki/Email_challenge-response

http://www.spamarrest.com/tryit/index.jsp?1339089174837

http://www.spamhelp.org/services/listings/challenge-response/

https://en.wikipedia.org/wiki/Challenge-response_spam_filtering

http://xeams.com/challengeresponse.htm

http://www.junkemailfilter.com/spam/working_with_our_competitors.html

http://www.google.com/postini/