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Challenge / Response Spam Filtering
Only mail from known senders gets through, and new senders must prove they are human (instead of spambots). That's the simple concept that makes challenge/response spam filtering effective. Find out all you need to know with these links, articles and software reviews.
What You Need to Know About Challenge/Response Spam Filters
How to get rid of spam. If this is the challenge, challenge/response systems are an elegant and efficient response. Find out how challenge/response spam filters work.