To resolve this warning, follow these steps:
Increase diagnostic logging so that you can determine which attributes or properties contain invalid values. Specifically, increase the diagnostic logging level on the Validation counter of the MSExchange ADAccess performance object. You can increase logging by running the following Exchange Management Shell cmdlet: Set-EventLogLevel "MSExchange ADAccess\Validation" -Level High. For detailed information about how to increase logging, see How to Change Logging Levels for Exchange Processes.
You can also change the logging level by editing the registry as described here:
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.
Start Registry Editor (regedit).
Locate the following registry key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchange ADAccess\Diagnostics
In the results pane, change the Value data of the General key to 5.
Exit Registry Editor.
Important After you have completed troubleshooting this issue, return logging to the default level by running the following cmdlet: Set-EventLogLevel "MSExchange ADAccess\Validation" -Level Lowest.
Review the additional MSExchangeADAccess events in the Application log to determine which attributes or properties contain invalid values.
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.