Is your Outlook keep poping up with authentication window? Is Outlook asks for username/password everytime it opens?
If you or the user changed the password (AD account) recently, Outlook authentication issue is bound to happen. I forgot where I read it (forums or MSKB), there is a simple solution. Changing your password while Outlook is running online mode resolves the issue on my experience.
1. Open the Outlook and type the username/password if asks.
2. Click Control-Alt-Delete key on your keyboard. Click on "Change Password"
3. Type your new password.
4. From next time Outlook will not challenge you for authentication.