This Error event indicates that the Microsoft Exchange Availability service could not successfully send a proxy Web request to another instance of the Exchange Availability service that is running in a different Active Directory site or a different Active Directory forest. The Exchange Availability service retrieves the Schedule+ Free Busy and Out-of-Office (OOF) data for a set of mailboxes that resides on a computer that is running Exchange Server 2007 Server or Exchange Server 2010. This event may occur when an Exchange 2007 or Exchange 2010 Client Access server and user mailbox are in different Active Directory sites or forests. Because the Exchange Availability service cannot directly connect to a mailbox that resides in a remote Active Directory forest, it sends a proxy Web request to another instance of the Exchange Availability service that is running in the remote Active Directory forest. The Exchange Availability service that is running in the remote Active Directory forest retrieves the requested information locally and passes the information back to the Exchange Availability service that requested the information.

This event may occur if one or more of the following conditions are true:

• The Active Directory site or forest that contains the user mailbox does not have a local Exchange 2007 or Exchange 2010 server that runs the Exchange Availability service.
  
  
• The Exchange Availability service finds issues when it tries to connect to the remote Active Directory forest.
  
  
• There are insufficient permissions to request data from the remote Active Directory forest.
  
  

To resolve this error, do one or more of the following:

• Make sure that the Active Directory site or forest that contains the user mailbox has at least one local Exchange 2007 or Exchange 2010 server that runs the Exchange Availability service.
  
  
• Use the Ping or PathPing command-line tools to test basic connectivity. Use Ping to isolate network hardware problems and incompatible configurations. Use PathPing to detect packet loss over multiple-hop trips. For more information, see Microsoft Knowledge Base article 325487, How to troubleshoot network connectivity problems.
  
  
• Increase the diagnostic logging level of the MSExchange Availability service. To do this, follow the steps shown here:
  
  Caution   Incorrectly editing the registry can cause serious problems that may require you to reinstall your operating system. Problems resulting from editing the registry incorrectly may not be able to be resolved. Before editing the registry, back up any valuable data.
  
  
  1. Start Registry Editor (regedit).
     
     
  2. Locate the following registry key: HKEY_LOCAL_MACHINE\CurrentControlSet\Services\MSExchange Availability\Diagnostics
     
     
  3. In the results pane, change the Value data of all keys (except Default) to 5.
     
     
  4. Exit Registry Editor.
     
     
• Review other related Warning events and Error events in the Application and System log. These related events may help you find the root cause of this event.
  
  

If you cannot resolve this error using the options suggested here, contact Microsoft Product Support Services. For more information, visit the Microsoft Help and Support Web site.

If you are not already doing so, consider running the Exchange tools created to help you analyze and troubleshoot your Exchange environment. These tools can help make sure that your configuration aligns with Microsoft best practices. They can also help you identify and resolve performance issues, improve mail flow, and better manage disaster recovery scenarios. To run these tools, go to the Toolbox node of the Exchange Management Console. To learn more about these tools, see Managing Tools in the Toolbox.