An auto-responder email is a message that's sent back instantly to everybody who sends an e-mail to an e-mail address with this feature enabled. The senders will get the automatic answer the moment their messages are received and you'll not need to do anything manually. A few instances of using auto-responder messages are mailing a validation that a payment is received and also the order is now being processed or that an individual is on holiday and will not be available for a specific time period. In the first case the option will be activated for a long time, while in the second one it's short-term. Even if you make use of email mainly for personal communication, you can use an auto-responder to let the email sender know that the e-mail is received successfully.