You may encounter a long delay in receiving email messages by using Microsoft Outlook in a Microsoft Exchange Server 2010 environment. However, you receive the email messages timely when you use Outlook Web App (OWA).
This issue occurs because of a deadlock that occurs in email notification threads in the Microsoft Exchange RPC Client Access service.
To resolve this issue, install Exchange Server 2010 Service Pack 3 (SP3) on the Exchange Server 2010 servers. For more information about Exchange Server 2010 SP3, click the following article number to view the article in the Microsoft Knowledge Base:
2808208 Description of Exchange Server 2010 Service Pack 3
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.