Make sure cached exchange mode is enabled.
Specifically this bit…
Method 1: Repair the Exchange email account
To repair the Exchange email account, follow these steps:
Click File, Account Settings, and then click Account Settings.
With the Exchange email account selected, click Repair.
In the Repair Account dialog box, click Next.
After the configuration finishes, restart Outlook.
After you restart Outlook, the status bar may show your status as Working Offline. To connect, follow these steps:
Click the Send/Receive tab.
Click Work Offline to turn off this feature.