I had this issue for a while after installing Exchange 2007 whereby users would always get a cert error whenever they launched Outlook.
If you have installed Exchange 2007 Client Access Servers in your organization, and if you have installed your SSL certificates (even commercial ones) on IIS, Outlook MAPI users may receive ‘Security Alert’ messages similar to the above in Outlook.
The name on the security certificate is invalid or does not match the name of the site.
This is because of the certificate that you have installed on IIS. Outlook 2007 MAPI clients use Client Access Servers for the Autodiscover service. The Autodiscovery web service (a virtual directory on the Client Access Server) is used for automatically finding the mailbox server for a given user. When the Autodiscover service is accessed by Outlook, and the name on the security certificate installed in IIS doesn’t match the internal FQDN of the Client Access server (CAS), this error results.
Suppose your company’s public domain name is mycompany.com. You may have obtained a certificate for webmail.mycompany.com and installed on the IIS of your Client Access Server. This is correct because users on the internet will type the public name.
However, the same IIS on the CAS is hosts the Autodiscover virtual directory as well and this certificate applies. Your internal domain name might be mycmpny.local and the client access server FQDN might be CAS1.mycmpny.local. Outlooks 2007 uses this internal name to connect to Autodiscovery, and hence the mismatch error.
To fix this problem, open Exchange Management Shell and type the following commands:
Set-ClientAccessServer -Identity CAS1 -AutodiscoverServiceInternalUri https://webmail.mycompany.com/autodiscover/autodiscover.xml
Set-WebServicesVirtualDirectory -Identity “CAS1\EWS (Default Web Site)” -InternalUrl https://webmail.mycompany.com/ews/exchange.asmx
Set-OABVirtualDirectory -Identity “CAS1\oab (Default Web Site)” -InternalUrlhttps://webmail.mycompany.com/oab
Set-UMVirtualDirectory -Identity “CAS1\unifiedmessaging (Default Web Site)” -InternalUrl https://webmail.mycompany.com/unifiedmessaging/service.asmx
Pay attention to the text in red, you will need to change it to reflect your server’s running parameters. Recycle the MSExchangeAutodiscoverAppPool or run iisreset. Your users should no longer receive the security alert.