You can use autoresponders to send a message back automatically to anyone who sends an email to a certain account. This can be useful for times when you are on vacation or unavailable, or if you have a generic message that you wish to send for a support email address.
Use this wisely. If you receive a lot of spam and have autoresponder active, you could be black listed as a spammer for send all that spam back. Try not to use it as a common practice to every email sent to you.