This Error event indicates that Microsoft Office Outlook Web App could not proxy a request from a computer that is running the Client Access server role to a Client Access server that is located in a different Active Directory site.

To try to resolve this issue, Exchange proxied the request to an alternative Client Access server in the same Active Directory site as the Client Access server that failed. However, no Client Access servers respond in that alternative site. A symptom you may experience when this event is logged is that some users may be unable to access their mailboxes using Outlook Web App through this Client Access server. This event may be caused by network connectivity issues between this Client Access server and the remote Client Access server.

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

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

• 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.
  
  
• Review the Application and System event logs for network-card-related events or connectivity-related events.
  
  
• Check to see whether other computers that use the same default gateway and are plugged into the same hub or switch have also experienced connectivity problems. If the other computers connected to the same default gateway have not experienced network connectivity problems, the problem may be caused by a network adapter issue on a single computer. If this is the case, try to resolve this issue by following one or more of these steps:
  
  
  • Replace the network adapter with a network adapter that has been tested and proven reliable.
    
    
  • Update the driver for the network adapter driver to the latest version.
    
    
  • Replace the network cable between the system that failed and the hub or switch.
    
    
• Check the settings on the network adapter and uplink hardware. The uplink hardware is the hub or switch that is used by the computer that has the Unified Messaging server role installed. Make sure that all network hardware is configured correctly by doing the following:
  
  
  • Make sure that the speed and duplex-level settings are the same on all network hardware.
    
    
  • If the media type is set to autosense, autosensing, autodetect, or Auto Select, make sure that all components autosense correctly.
    
    
• 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.