An auto-responder email is an e-mail that is sent back immediately to everyone who sends an e-mail to an e-mail address with this feature enabled. The senders will get the automatic answer the instant their e-mails are received and you will not need to do anything whatsoever by hand. A couple of examples of making use of auto-responder messages are mailing a confirmation that a payment has been received and the order will be processed or that an individual is on vacation and won't be available for a specific period of time. In the first case the feature will be activated permanently, while in the second one it's non permanent. Even if you make use of email for personal correspondence, you may use an auto-responder to let the email sender know that the e-mail is received successfully.