The auto-responder is a feature that will automatically respond to incoming e-mails with a message that you create. This is convenient when vacationing as you can set the auto-responder message to inform others that due to vacation (or another obligation), you will not be able to check your e-mail until a specific date when you return such as:
Unfortunately I will be unable to check my e-mail from Tuesday March 8th until Monday March 14th.
If you have any urgent matters, please contact Mary at Mary@Jane.com or (123) 456-7890.
This way, if someone forwards you 15 e-mails in one day, they will only be informed the first time of your auto-responder message and not receive 15 replies. If they were to send you 2 e-mails the next day, they would still only receive the auto-responder message once for that day.