Question about Outlook Express

Hi Everyone, 

I have an interesting issue with Outlook 2011 for Mac and my Work Exchange. 

I set-up the account and it works fine for a few days. Then, it starts giving me a login failure, telling me that my credentials aren't good. I re-enter the same password, and it starts working again. Then suddenly one day, I try re-entering my password and Outlook keeps rejecting my credentials, until I restart my computer. After that, it's smooth sailing until the process repeats. 

Any ideas as to what might be causing this? 

Thanks, 

Peter