An e-mail auto-responder is a an e-mail mailbox which automatically sends a pre-configured message back to all senders immediately upon receipt of their message.
When you configure an auto-responder, any messages received by that mailbox will automatically trigger a return message containing your pre-configured auto-response. This is particularly useful for mailboxes such as info@yourdomain.com or sales@yourdomain.com. Every time a customer sends you an e-mail message requesting sales information, you can automatically send them back a "canned" response containing information and answers to most common sales questions. This is helpful in that your customer receives an immediate response 24 hours a day.