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.