When an Outlook 2007 user has their mailbox migrated from Exchange 2003 to Exchange 2007 they may start seeing a certificate mismatch error appear when launching Outlook.  This problem will occur if the following conditions are true:

• the internal DNS namespace does not match the external DNS namespace (eg domain.local for internal and domain.com.au for external)
• a third party SSL certificate matching the external namespace has been applied to the IIS instance on the Client Access Server (eg cas.domain.com.au)

The certificate mismatch occurs because the Autodiscover Service and/or Availability Service use the FQDN of the Exchange 2007 host in their URL (eg https://server.domain.local/AutoDiscover/AutoDiscover.xml) but the IIS instance has an SSL certificate that does not match that name.

To resolve this issue you can change the URL of the Autodiscover and Availablity services to match the name on the certificate, which you can do with the following shell commands.

[PS] C:\>Set-ClientAccessServer -Identity cas.domain.local -AutoDiscoverServiceInternalUri https://cas.domain.com.au/Autodiscover/Autodiscover.xml

[PS] C:\>Set-WebServicesVirtualDirectory -Identity "EWS (Default Web Site)" -InternalUrl https://cas.domain.com.au/EWS/Exchange.asmx -ExternalUrl https://cas.domain.com.au/EWS/Exchange.asmx

As long as your internal clients are resolving the external name cas.domain.com.au directly to your Client Access Server this should resolve the certificate mismatch error.