An active autoresponder answers to an incoming e-mail with desired message, automatically. This function is useful if you are not able to read your mails (because you are for example on holiday) and you want to inform every sender.
You will find this option via [Webhosting] → [EMail Addresses] when you edit an address.
If you want to set up your autoresponder, log into your control panel. Click on [Web Hosting] and then [Email Addresses]. Now click on the edit icon of the respective Email Address for which you want to set up an autoresponder. Once you click Enable the checkbox next to "Autoresponder", more input options will appear. Enter the desired subject in  "Subject:" and write in the field "Content:" the desired message. After you have clicked on "save" your autoresponder for the desired e-mail address will be activated.