Outlook:Mac 2011 deletes email on exchange after downloading it

I'm asking this question on behalf of one of our Office:Mac users. After installing and setting up Outlook to connect to the Exchange server in the office, the user is complaining about a rather strange issue. Outlook is behaving as if it was interacting with a POP account, namely deleting server messages after they have been downloaded on the Mac. This becomes apparent when emails come in and are seen on his PC and Phone, but as soon as the Mac receives the email, the message disappears from the server and is only available locally on the Mac. 

I've installed Office:Mac 2011 on another Mac computer to see if I can duplicate the issue, but this installation behaves as you would expect. I can't figure out what the problem is. The Outlook that's acting up is set up straight out of the box with all default settings and connected to one Exchange account. The On My Computer folder is hidden, there are no rules to delete messages on the server, and there doesn't seem to be any reason for it to do what it's doing.