An anti-spam program that requires a human being on the sender’s end to respond to an emailed challenge message before their messages can be delivered to recipients. Senders who answer the challenge successfully are added to an authorization list. Bulk emailers can work with challenge-response if they designate an employee to watch the sending address’ mailbox and to reply to each challenge by hand.

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You finally realize that you need a good opt-in list. After reading countless articles and sought expert advices and have read many success stories of people creating a small fortune with opt-in lists you finally decide to have one of your own.