This Error event indicates that a proxy Web request failed one or more security checks at the Web service layer. When the Microsoft Exchange Availability service cannot directly connect to a mailbox that resides in a remote Active Directory site or forest, it sends a proxy Web request to another instance of the Exchange Availability service that is running in the remote Active Directory. The Exchange Availability service that is running in the remote Active Directory site retrieves the requested information locally and passes the information back to the Exchange Availability service that requested the information. This event occurs when the Exchange Availability service that requested the information cannot be authenticated by the receiving Availability service. This event is logged on the Exchange server that receives the Web request from a remote Active Directory site or forest. 

To resolve this error, 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.