Ever since day one at Adobe I've had an odd issue with Outlook on my Mac. About 3-4 times a day it would 'forget' my password. As soon as I entered my password it worked, but it bugged me that I had to keep re-entering it. I Googled but never found the right answer. I deleted and recreated my profile numerous times, but it never helped.
Finally, Terry White (fellow Adobe evangelist!) made a simple suggest. Instead of entering my password in Outlook, I should be doing it in the Exchange settings via System Preferences.
So first you go here...
And then click Details:
That was it. Seriously. I can't believe I didn't figure that out earlier, but every single time I tried to fix it by Googling around I never came across this solution. (Probably because it was too obvious. ;)