This first happened to me a day after setting up Entourage but for some people it randomly happens after several months.
Symptoms: You have Entourage 2008, Tiger or Leopard connected to a Microsoft Exchange Server. You are getting mail however your inbox will not update to show any of these emails.
Fix: Open Entourage:
1. Right click on inbox
2. Select Folder Properties
3. Click Empty under Empty Cache
Your Inbox will then be downloaded from the Exchange server and should be updated with all your new emails.
I’m not sure why this happens however it most likely has something to do with a loss of connection to the exchange server (in my case because the VPN went down). When the connection is restored for some reason Entourage won’t update the inbox.