When an email is sent to a certain address, an automated message or sequence of messages is delivered. An out-of-office email, for example, is an auto-respondent.

You may use an auto-responder to deliver email messages to persons who have chosen to receive them (your subscribers). For instance, if you’ve written a free report, template, guide, or other useful pieces of material, you may make it available to your website visitors in return for their email address.