Autoresponder
Autoresponder automatically responds to anyone that sends mail to it. These are used for a few things, including vacation notices and easy dissemination of information.
If you had a vacation autoresponder for instance, you could turn it on when you leave, and anyone that sends mail to your address while you're gone will get an immediate response stating that you are out of town. Businesses use Autoresponders for getting sale information, new product announcements, e-mail brochures and more out to interested customers.

