Process %1 (PID=%2). Exchange Active Directory Provider received a request to connection to domain controller %3 but that domain controller is not available. Use the Ping or PathPing command-line tools to test network connectivity to local domain controllers. Run the Dcdiag command line tool to test domain controller health.

This Error event indicates that the domain controller specified in the event description is not working or not reachable. The Active Directory® provider maintains a pool of connections to the available domain controllers. Any Exchange process that has to access Active Directory uses an available connection from the connection pool. This event may occur if the connection in the connection pool indicates that the domain controller is not working or not reachable.

To resolve this error, do the following:

1. Make sure that the domain controller is running and operational. Do one or more of the following:

   • Review the Application log for MSExchangeADAccess Event ID 2150. Event ID 2150 provides more information about the available domain controllers, global catalog servers, and configuration domain controllers in the Active Directory provider connection pool.

   • 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.

   • Run the Dcdiag command line tool to test domain controller health. To do this, run dcdiag /s:<Domain Controller Name> at a command prompt on the Exchange Server. Use the output of Dcdiag to discover the root cause of any failures or warnings that it reports. For more information, see Dcdiag Overview at the Microsoft Windows Server TechCenter.

2. After you make sure that domain controller specified in the event description is up and running, restart the Active Directory topology service to rebuild the connection pool.

If this error persists, review the Application log and System log on the domain controller and Exchange server for any corresponding Warning or Error events.

If you are not already doing so, consider running the tools that Microsoft Exchange offers to help administrators analyze and troubleshoot their Exchange environment. These tools can help you make sure that your configuration is in line with Microsoft best practices. They can also help you identify and resolve performance issues, improve mail flow, and better manage disaster recovery scenarios. Go to the Toolbox node of the Exchange Management Console to run these tools now. For more information about these tools, see Toolbox in the Exchange Server 2007 Help.