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These systems are commonplace programs that verifies email messages from unknown senders. These responses issue the sender a simple CAPTCHA test, which are visual-based tests designed to differentiate human senders from automated senders, more commonly referred to as “bots.” These challenge-response systems ensure that emails and important messages from human senders are channeled through to a user’s inbox while spam messages are rejected.
Most commonly, challenge-response systems will send an email with a link to a URL. This website link will direct the sender to perform a task that is simple for humans, but impossible for spam bots. Often, the sender will be asked to type in the letters or number from an image, or to answer a basic arithmetic question such as “2 + 2 = ?” Most web-users are familiar with these systems. Many companies use these programs when they allow users to create free accounts.
Once a sender has been subjected to their first CAPTCHA test, the sender’s address is whitelisted and become accepted mailers, which means that senders do not have to verify their messages multiple times.
