When you use a wildcard certificate on your Exchange 2007 or 2010 environment you can receive an error on outlook:
There is a problem with the proxy servers's security certificate.
The name on the security certificate is invalid or does not match the name of the target site mail.domain.com
Outlook is unable to connect to the proxy server (Error code 0)
When using internally your outlook may connect just fine using the normal RPC-TCP method.
This happens because outlook is checking the name on the certificate for mutual authentication to ensure your are connecting to the right server. Outlook gets this information from the autodiscover service.
If you manually change the value to msstd:*.domain.com it works, but the autodiscover will put the other value back in a matter of minutes. Autodiscover assumes a value equal to the external name set on your CAS server (in my case mail.domain.com) and uses this.
To override this behavior use the following exchange shell command:
Set-OutlookProvider EXPR -CertPrincipalName msstd:*.domain.com
After you adjust this, you need to restart the “world wide web publishing” service, because of caching.
You could also disable this “Mutual authentication”, but it’s a good security feature, so I wouldn’t.
Set-OutlookProvider EXPR -CertPrincipalName none
Remember to restart the W3P service.
Be aware that when you set $null instead of none Exchange will go back to default behavior and use the external name from the CAS server.