I have a test environment with the usual 2008 R2 DC & Exchange LAN setup, secured by TMG 2010.
We also have a production environment with Sharepoint, Exchange etc hosting an intranet and Exchange OWA; all of this is also secured by TMG 2010.
Both environments are on seperate internet lines & ISPs and are not connected in any form or shape. There is also NO VPN access from one environment into the other so they are completely seperate.
In the test environment I have full internet access and can access the internet in our production environment, let's call it https://www.domain.com but I am unable to connect to the OWA subdomain https://webmail.domain.com
There is however nothing wrong with external access to OWA as I can access it from various other PCs and smartphones, proofing that the OWA setup works fine externally and internally.
The only thing I can think of is that the TMG firewall in our production environment somehow blocks requests that originate from our test environment.