Challenge-response
System demanding sender confirmation before delivering email
Challenge-response is a system used to filter spam email by requesting confirmation from senders of unsolicited emails.
The sender is sent a mail requesting they take some action (such as clicking on a link and complete a CAPTCHA) to confirm that they sent the mail. If the sender does respond the system judges them to be a real person, rather than a spamming bot, and passes the mail on to the intended recipient, often adding the sender to a whitelist.
The system has been criticised for adding to the drain on network bandwidth caused by spam, as often spam will use a forged header with fake response details, so innocent people end up being 'spammed' by the challenge-response system, querying emails they never sent.
