Assume that customized Exchange transport agents are deployed in a Microsoft Exchange Server 2010 environment. When you try to upgrade Exchange Server 2010 to Service Pack 1 (SP1) or a later service pack, the transport agents are not loaded. Additionally, an event that resembles the following is logged in the Application log:
This issue occurs because the transport assemblies are updated in each service pack for Exchange Server 2010. For example, the Microsoft.Exchange.Data.Transport and Microsoft.Exchange.Data.Common assemblies are updated.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.