You may recieve the below error when try to access the OWA (Outlook Web App) published through Microsoft TMG 2010. The Outlook Web App (OWA) login page will appear without an issue, but after providing the username and password you will recieve website can not be found error.
Technical Information (for support personal)
Error Code 11001: host not found
How to Fix:
As the error indicates, this issues happens because the TMG server unable to reach the exchange server. It mostly related to the DNS resolution issue as you might have given the Exchange Server name in "this rule applies to this published site" section in "To" tab of the publishing policy.
To verify the error is due to it, try testing the publishing rule
You may also create a host file entry to resolve the issue quickly, before you fix any major DNS issues resolution between TMG and internal DNS servers(due to any firewall/network configs).
Ensure that all the required ports are allowed and no network configurations are changes between TMG and Internal Exchange CAS as normally the TMG sits in the DMZ network.