"OOA is available only if you connect to Microsoft Exchange Server. It sends a response (just once) to each message sent to you and also executes other rules that you set up in OOA. Always fill in the response text; otherwise, your correspondents will receive a blank message in return."
Read neat the end for a workaround at:
When using Outlook, I am unable to set up my Out of Office Assistant. The selection is not listed in the Tools drop down menu. Any ideas?