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Challenge/Response Spam Filters

Challenge/Response filtering (also called "autowhitelisting") is not a panacea. It transfers disruption to those emailing you, including those who you wish to contact you. Poor implementations reply to list-based email. Try to avoid using this kind of method if you can.

Best Practice for Challenge/Response

If you want to implement challenge/response to filter spam you should read these pages.

Proper principles for Challenge/Response anti-spam systems - www.templetons.com/brad/spam/challengeresponse.html
Challenge/Response at the SMTP Level - jamesthornton.com/writing/challenge-response-at-smtp-level.html
Challenge / Response Interworking (CRI) Framework - www.ietf.org/internet-drafts/draft-irtf-asrg-cri-00.txt

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Challenge/Response Considered Harmful

These are the reasons why you shouldn't use challenge/response.

John Levine: Challenge-response systems are as harmful as spam - www.politechbot.com/p-04746.html
A Challenging Response to Challenge-Response - www.freedom-to-tinker.com/archives/000389.html
Challenge-Response Anti-Spam Systems Considered Harmful - kmself.home.netcom.com/Rants/challenge-response.html
How Challenge/Response Spam Filters Work - email.about.com/cs/spamgeneral/a/challenge_resp.htm
Inaccessibility of Visually-Oriented Anti-Robot Tests - www.w3.org/TR/2003/WD-turingtest-20031105/

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Challenge/Response Intellectual Property Issues

Challenge/Response Intellectual Property Issues

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Challenge/Response Articles by Marketers

Challenge-Response: Not the Challenge that Legitimate Marketers May Think - www.silverpop.com/resource_white_papers.shtml
Challenge-Response What it means for legitimate email marketers - www.digitalimpact.com/newsletter/sept03-challengeresponse.html
Responding to Another E-Mail Challenge - www.clickz.com/experts/brand/buzz/article.php/2217851

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Challenge/Response Spam Filters

Any spam filter implementation that uses a challenge/response technique that is based on sending challenge messages to an address taken from the email envelope or headers is listed on this page in preference to the other pages on this site.

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Challenge/Response Techniques

To Err Is Not Always Human - research.microsoft.com/displayArticle.aspx?id=413
Petmail - petmail.lothar.com

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Perl Client Side Challenge/Response Spam Filters

BLOWBACK - www-2.cs.cmu.edu/~sleator/blowback/

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Third Party Delete from POP3: Challenge/Response

CleanMyMailbox - www.cleanmymailbox.com (Reviews: 1)
Atqui - www.atqui.com
Spam Wolf - spamwolf.com
GoodbyeSpam - https://www.goodbyespam.com/
iPermitMail - www.ipermitmail.com
The Spam Eater - thespameater.com
0Spam - www.0spam.com
MailKey - www.mailkey.com
SpamFry - www.spamfry.net
SpamClear - www.spamclear.com
Bluebottle - www.bluebottle.com

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Spam Filtered Challenge/Response Accounts

MailCircuit - www.mailcircuit.com (Reviews: 1)
Email Bouncer - www.email-bouncer.com
USOpt - www.usopt.com
X-Mail - www.x-mail.net
MailKey - www.mailkey.com
myprivacy .ca - www.myprivacy.ca - for .ca domain registrations
Email Validation Service - www.evsmail.com
Without Spam - www.withoutspam.com
SpamSucks - www.spamsucks.com/spam/
Spam-Stopper - www.spam-stopper.co.uk
Spam Wall - www.spamwall.net
Expamintor - www.expaminator.com

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General Unix Client Side Challenge/Response Spam Filters

mapSoN - mapson.sourceforge.net
Active Spam Killer - www.paganini.net/ask/

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Windows Client Side Challenge/Response Spam Filters

AllSpamGone - www.allspamgone.com (Reviews: 1)
KnockKnock - www.knockmail.com (Reviews: 1)
ChoiceMail - www.digiportal.com/choicemail.html (Reviews: 1)
SpamBunker - spambunker.buddyshare.org
Steven - www.softwaredevelopment.net.au/pge_steven.htm
WinAntiSpam - www.winantispam.com
Spam Research Center - www.spamresearchcenter.com
Vanquish - www.vanquish.com
Mail Unknown - www.mailunknown.com
spamPepper - www.spampepper.com
SpamSlam - www.ilesa.com
PureMail - puremail.ironcanucks.com
MailSword - www.mensk.com/software/mailsword/mailsword.htm
Kens Spam Filter - www.gb7abc.net/spam.html

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Outlook Spam Filter Challenge/Response Plugins

SpamEnder - spamender.com (Reviews: 1)

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Server Challenge/Response Spam Filters

SpamLion - www.spamlion.com - ASP or gateway
InboxLock - www.inboxlock.com - mail server plugin or appliance
Antispam - www.spamkilling.com - Sendmail
Postmaster Pro - www.postmasterpro.com - Unix
CAPTCHA - www.captcha.net

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