I will be out of the office
This is the standard expression to write in an e-mail "out-of-office" message:
I will be out of the office.
You don't usually use "will" to talk about your future actions, but in more formal English you can. In more casual situations it's more common to use "going to". For example, if you tell a coworker about your vacation, a good way to explain it is:
I'm going to be out of the office next week.