If you are in an Exchange environment (ask your administrator if you do
notknow) you can set the Out of the Office Assistant which is located under
Tools.

If you are not in an Exchange environment you can create a rule that replies
on all e-mails. The downside of this is that you'll have to have your
Outlook open all the time for the rules to process. Make sure you set some
exceptions (for instance based on the subject field so it will not reply on
subject with e.g. reply, failed, undeliverable, etc...) otherwhise you could
create endless mailloops between two mailservers and those can be a real
threat to the mailservers!