If you're in a business environment and connect to a Microsoft Exchange
server for email, your out of office is under tools. Also note that Exchange
will only respond once to each incoming email with the out of office message.
It will not respond every time.
If you have another type of mail account, you need to set up rules:
http://www.howto-outlook.com/faq/oooa.htm
You want to be careful responding to ALL email since this will provide your
email address to spammers.