This Warning event indicates that the computer that is running the Client Access (CAS) server role could not determine the Client Access server that should handle a Microsoft Office Outlook Web App request. Specifically, it could not determine whether the request should be processed by the local Client Access server, or if it should be proxied or redirected to a Client Access server in a different Active Directory site. Therefore, the mailbox specified in the event description could not be accessed through Outlook Web App. This event may be caused by one or more of the following:

• The Client Access server cannot verify the location of servers that are running Microsoft Exchange in Active Directory sites.
  
  
• The Client Access server cannot verify the links between the Active Directory sites.
  
  
• The Client Access server cannot access required Exchange virtual directory configuration data from Active Directory.
  
  
• The Client Access server that accepts Outlook Web App proxy or redirection requests is not in the same Active Directory site as the computer that is running the Mailbox server role. 
  
  

For more information about Outlook Web App proxying and redirection, see Understanding Proxying and Redirection.

To resolve this error, take one or more of these steps:

• If this occurs frequently, check network connectivity between the Client Access Server and domain controllers. 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.
  
  
• Make sure that the InternalUrl of the Outlook Web App virtual directory exists and that it matches the fully qualified domain name (FQDN) of the local CAS server.
  
  
• Resolve your issue by using self-support options, assisted support options, and other resources. You can access these resources from the Exchange Server Solutions Center. From this page, click Self-Support Options in the navigation pane to use self-help options. Self-help options include searching the Microsoft Knowledge Base, posting a question at the Exchange Server forums, and others. Alternatively, in the navigation pane, you can click Assisted Support Options to contact a Microsoft support professional. Because your organization may have a specific procedure for directly contacting Microsoft Technical Support, be sure to review your organization's guidelines first.
  
  

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.