Please make this a plugin for Mozilla Thunderbird. It seems like it could easily be implemented by adding a couple of new actions to the list available for a message filter: Challenge Sender, and Add to Collected Addresses. If the sender is not in the 'Personal' or 'Collected Addresses' Address Book and the Challenge Reply header IS NOT set properly then Challenge the sender. If the sender is not in the 'Personal' or 'Collected Addresses' Address Book and the Challenge Reply header IS set properly then add the sender to the 'Collected Addresses' Address Book. You could also add an action to 'Black List' any addresses that are challenged but not authenticated within a certain period of time. For the optional 'Black List' add a rule that simulates a Mailer-Daemon response if the sender is in the 'Black List' Address book. btw, excellent article.

Recently read about a similar idea, but fully automated. The receiving MTA rejects e-mail from "new sources" with a 4xx error. As this is defined in SMTP to mean "temporary problem, please try again later", any legitimate MTA will retry again later. Spammers almost always use fire-and-forget MTAs that dispense with the niceties of SMTP including retries. I've simplified the idea somewhat. They call the approach "greylisting" and there's a paper out on it at: http://projects.puremagic.com/greylisting/whitepaper.html I found out about it through Ted Leung's weblog: http://www.sauria.com/blog/2004/07/05#1004

Great idea, but what about the Internet SMTP traffic incresed by 2 by "bots" that responds to the Challenge automatically. If they manipulate the "Reply-to:" or "Return-Path:" header, and the software doesn't care, it could happen.

Hmm, this might actually really work some time in the near future. Comparing your article with some of the earlier musings/suggestions/articles about CRS shows some real progress. But a few questions popped up during a quick read though: 1. The already mentioned bot-automated responses 2. If you have to whitelist all mailing lists you'll still need a filter for messages sent to those mailing lists (appr. half of the spam in my Inbox/Junk folder is sent to mailing lists, half directly to my address). 3. If you don't have your own server changing your ISP/equivalent can become a pain (This currently applies to filters as well). But please do keep thinking and experimenting.

Except there's still a big problem. Many times I get emails with forged from addresses. What this means is that I would be sending spam back out to the poor bastard that has had their reply-to address forged.

Has anyone of you looked at TMDA (http://www.tmda.net)? It does whitelists, blacklists, challenge/response and so called tagged addresses.

What about registering with web sites? Typically they send an authorization email (sometimes with your password in). You won't know what to add to your white list.

As noted by many other spam researchers, this C/R technique: 1- doubles the amount of spam mail (since for every spam you respond with a challenge) 2- assumes the first email isn't spoofed -- it's unreliable 3- only works if the person doesn't use a C/R system of their own -- it doesn't scale. Three fatal flaws IMHO. http://kmself.home.netcom.com/Rants/challenge-response.html http://www.politechbot.com/p-04746.html I like the greylisting idea a whole lot better. "Challenge systems have effects a lot like spam. In both cases, if only a few people use them they're annoying because they unfairly offload the perpetrator's costs on other people, but in small quantities it's not a big hassle to deal with. As the amount of each goes up, the hassle factor rapidly escalates and it becomes harder and harder for everyone else to use e-mail at all."

An interesting take could be using the spammer technique of placing an html link to a picture with a unique id the would send back a ping identifying the user just by the recipient opening the email. This would take more responsibility away from the recipient. It is true that everybody doesn't have HTML email, so that you would have to still allow replies to validate email. The possibility of the emailer using someone else's email address would have a more drastic effect with this idea also, being the email would just have to be opened, a positive response would be given even if the person really wasn't the sender of the email. But it is still an interesting twist on the idea.

What stops a spammer from automatically responding with an empty reply and being validated?

I noticed that some spams now use my own email address as the sender, which makes the implementation of the CRS more complicated...

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To get rid of cancer you nearly have to kill the patient. Maybe with spam the same is true. Clog the web pipes with more C/R messages. Bring it to its knees. If sender/return addresses are different then it is likely spam or a friend on vacation. Known baddies are purged but review new messages daily. Black lists could get *very* long but after a while entries are probably discontinued and could be dropped for others. Remember private/public encryption? Could that approach work here?

Are there any programs available for Windows that intercept the email (LSP) before it gets to the email client? Email client independent?

Todd: I am no geek am not sure I really understand you!! But please take a look at www.choicemail.com. This has done a good job for me on one account. I dont want to pay for the full edition to cover my others. I can still take a quick check on spam, but it doesnt ever reach my PC unless I give it the okay. Spammers never respond to a challenge - at least mine dont! KR Denis

Todd: I am no geek am not sure I really understand you!! But please take a look at www.choicemail.com. This has done a good job for me on one account. I dont want to pay for the full edition to cover my others. I can still take a quick check on spam, but it doesnt ever reach my PC unless I give it the okay. Spammers never respond to a challenge - at least mine dont! KR Denis

Todd: I am no geek am not sure I really understand you!! But please take a look at www.choicemail.com. This has done a good job for me on one account. I dont want to pay for the full edition to cover my others. I can still take a quick check on spam, but it doesnt ever reach my PC unless I give it the okay. Spammers never respond to a challenge - at least mine dont! KR Denis

I have been also been using a challenge response system ( http://www.ixxorealmail.de ) which effectively stopped all the spam coming to my inbox. I beleive that challenge response systems are much better than any filtering system, at least for me.

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