To resolve this warning, you must use the New-ExchangeCertificate cmdlet to create a new TLS certificate for the computer that returned this Error event. For more information, see Creating a Certificate or Certificate Request for TLS.
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.