Definition: auto-responder

A program that responds to an email or other inquiry without human intervention. For example, if an email address is no longer valid, an auto-responder in the mail server sends an "undeliverable message" automatically to the sender. If the recipient has a new email address or is on vacation, an auto-responder can be set up to reply with an appropriate message.

Many people have received an inordinate amount of auto-responder messages due to email viruses, which send messages to random addresses extracted from the address books of unsuspecting users. If an email address is out-of-date, the auto-responder sends an "undeliverable" message to the sender, who had no idea the message was being sent in the first place.