Autoresponder is a program or script running on the mail server which automatically replies to the emails received in an account. Though autoresponders run on mail server they can be set up by using web based interface. Autoresponders can be set, when you are going for a vacation and cannot respond to emails for some time. Then you can set the autoresponder which will reply to the people emailing you that you are on a vacation and will reply as soon as you come back. Also, companies set autoresponders for support requests telling the visitors that they have received their message and will reply as soon as possible.