An auto-responder e-mail is an e-mail which is sent back immediately to everybody who sends an e-mail to an e-mail address with this feature enabled. The senders will receive the automatic answer when their emails are received and you will not have to do anything whatsoever manually. A couple of instances of using auto-responder e-mails are sending a validation that a transaction has been received and the order is being processed or that an individual is on a break and will not be around for a particular time frame. In the first case the function is activated for a long time, while in the second one it's non permanent. Even if you use an email address simply for personal correspondence, you may use an auto-responder to let the sender know that the e-mail has been received successfully.