Any fix for mail stuck in outbox in Mail 5.1 (using Exchange Server 2010)?

Every couple of days I will notice that mail hasn't been sending from my Exchange email account in Apple's Mail 5.1. There doesn't seem to be any issues on the server side so it is probably something on my side of things.

I have checked my settings with our Exchange Server representatives and everything should be fine. The 'stuck' messages will send only if I take the account offline temporarily, move the messages to a draft folder, bring the account back on line and then drop the messages one by one back into the outbox.

First, I had to 'unhide' the Outbox just to notice the problem. Now, the outbox doesn't even show the unsent messages unless I take the account offline temporarily.

Anyone find a solution to this?