An autoresponder is a computer program that automatically answers e-mail sent to it. They can be very simple or quite complex. The first autoresponders were created within mail transfer agents that found they could not deliver an e-mail to a given address. These create bounce messages such as "your e-mail could not be delivered because..." type responses. Today's autoresponders need to be careful to not generate e-mail backscatter, which can result in the autoresponses being considered E-mail spam. Autoresponders are often used as e-mail marketing tools, to immediately provide information to their prospective customers and then follow-up with them at preset time intervals. Such follow-up autoresponders can be divided into two categories: Autoresponders are also incorporated into electronic mailing list software, to confirm subscriptions, unsubscriptions, posts, and other list activities.
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