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:
Outsourced ASP model – these autoresponders operate on the provider's infrastructure and are usually configurable via a web-based control panel. The customer pays a monthly usage fee. This is easiest to implement for the end-user.
Server-side – enables users to install the autoresponder system on their own server. This requires technical skills.
Autoresponders are also incorporated into electronic mailing list software, to confirm subscriptions, unsubscriptions, posts, and other list activities. Popular email clients such as Microsoft Outlook and Gmail contain features to allow users to create autoresponses.