It refers to a mechanism that responds to incoming emails automatically.

There is a standard way how autoresponders react to the emails received to a given email address. There is however few autoresponder’s that are custom built to respond to emails sent in a way user has defined it to. The mechanism involved in an autoresponder can either be simple or complex depending upon number of factors. There are basically two types of autoresponders. While one of them is based on Server-side model other is based on Outsourced ASP. There are also autoresponders that are included in electronic software of mailing lists to confirm posts, subscriptions and a whole set of other activities.