When you query free/busy data for external contacts in Microsoft Exchange Server 2007, the following event is logged consistently in the Application log:
Event Type: ErrorEvent Source: MSExchange AvailabilityEvent Category: Availability Service Event ID: 4011Description:Process <ID>[w3wp.exe:/LM/W3SVC/1/ROOT/EWS-<ID>]: Configuration for forest <external domain name> was not found in Active Directory. Run the Add-AvailabilityAddressSpace command in the Exchange Management Shell for an Exchange Server 2003 Active Directory forest.
Note: This Event 4011 may cause confusing to the Exchange administrators because it is a normal situation that the Free/Busy data of external contacts is not available
This issue occurs because of a logic error in Exchange Server 2007. Specifically, Exchange Server 2007 tries to locate free/busy data for external contacts. When the free/busy data is unavailable, Exchange Server 2007 still logs an event. However, you do not expect Exchange Server 2007 to log an event when free/busy data for external contacts is unavailable.
To resolve this issue, apply the following update:
971534 Description of Update Rollup 1 for Exchange Server 2007 Service Pack 2
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.