Outlook 2011 – Message Unavailable

I have a single user using an iMac running OS X 10.7 and Office 2011. Multiple times a day he gets a message that says "System Administrator - Message Unavailable. The message you're trying to view is no longer available on the server." I see that as a simple error stating the message has been deleted, or moved, or there may be a synching problem. As a test I made sure to not make any changes to the mailbox for a period of two weeks. Regardless of any changes to the mailbox the user may get one message, or eighty in a day. I cannot find a single error/problem anywhere. So I'm a bit lost on what the possible cause may be. His mailboxes are not setup to auto delete/prune messages. I've tried some web searches but cannot find any solid information on the problem. Any advice would be great. Outlook version is 14.3.7