I currently have the Exchange Provider for EXPR set to $null, however I still seem to get msstd:mail.mydomain.com set in my Outlook connection string setting on all machines. Where is this setting coming from? We do push the Outlook Anywhere settings via GPO and have the ProxyServer string defined set to mail.mydomain.com. The flags are set to ensure SSL is defined on the connection.

We are in the process of moving from Exchange 2007 to Echange 2013. Will it be a problem moving from a UCC cert with a friendly name of mail.mydomain.com to a wild card SSL cert on the 2013 servers with *.mydomain.com set, if the "connect to proxy servers that have this principal name in their certificate is selected? I'm concerned that msstd:mail.mydomain.com  does not match msstd:*.mydomain.com?

Thanks in advance

With a wildcard certificate you should change the EXPR CertPrincipalName to msstd:*.domain.com.  It should work.