An auto-responder is an application which sends an automated email response to any (or specific) incoming emails.
These can be setup on the mail server, or on the user’s local computer (via programs such as Outlook). Autoresponders are most commonly used in business to alert senders that the recipient is not available to respond at that time, such as after hours, on weekends, or when they are on vacation. A typical auto-response might state something like “I am not at my desk right now, but will return on Monday”. As well as other uses for auto-responders, some companies set up unique email addresses that will send an automated response to any senders, with specific information such as a current price list.