A challenge–response (C/R) system is a type of filter that automatically sends a reply back with a request to the sender of an incoming email. In the reply, the sender is asked to perform an action to ensure delivery of the original message. The action to be performed is easy to do if you’re an individual, but requires a lot effort if performed in large numbers, in this way effectively filtering out spammers. A good example of this is a verification email where you need to click a link to verify your email address to be activated within a new system. This is important with an email system to verify that you are opting in to a system and to make sure that you’re not a spammer.
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