Challenge/Response: The 100 Percent Accurate Spam Filter
It has been several years since you could look for a specific word in e-mail to get rid of spam. Besides, to sneak past filters, spammers use different methods that change almost from hour to hour.
The most effective spam filters are permission-based tools, which block messages sent by anyone who isn't on your whitelist. That's great if your only wanted e-mail comes from friends and family members. But if you occasionally receive unexpected e-mail that you want to get, blocking every unknown sender won't work. That's where challenge/response, or CR, filtering apps come in. The term means that the filter will block e-mail from an unknown source unless the sender of the e-mail replies to a special message correctly.
Here's how it typically works: Right after your Uncle Sid sends you an e-mail, your CR tool sends him a message directing him to perform some action, such as clicking a link. If he responds correctly to the challenge, he gets added to the whitelist, and his messages get through from then on.
The challenge defeats spammers because virtually all junk mailers use bogus return addresses, and therefore can never correctly respond. But because the challenge demands a response, this type of tool can inconvenience the sender of the message. Afraid that a sales lead will e-mail someone else rather than go through the hassle of a response, businesspeople dread these kinds of tools even more than spam.
But for home users who bridle at the restrictions of a permission-based filter, and who dislike the hassle of training a rules-based filter, challenge/response filters can cut 100 percent of spam--and that's a diet that no one would mind sticking to.