An auto-responder email is an e-mail which is sent back instantly to everybody who sends an e-mail to an email address with this feature activated. The senders will get the automatic answer the moment their emails are received and you will not have to do anything by hand. A few instances of taking advantage of auto-responder emails are sending a validation that a payment is received and also the order is now being prepared or that a person is on a break and will not be available for a particular period. In the first situation the function will be activated for a long time, while in the second one it's non permanent. Even if you make use of e-mail address for private correspondence, you can use an auto-responder to let the sender know that the e-mail has been received successfully.