We will perform the following steps if we want our users not to have to write the /owa directory in Exchange, if we are using Microsoft Forefront Threat Management Gateway (TMG) 2010.

1º Create the rule to allow OWA to https://correo.midominio.com/owa (HTTP://www.bujarra.com/?p=4862).
2º Copy & Paste that rule into the TMG. Rename it and give it a correct name.
3º Edit it, from your properties:
– On the “Action” put 'Deny'’ and redirect http to 'https://correo.midominio.com/owa'.
– On the “Paths” delete all paths and create one with the following settings:
External path: <same as internal>
Internal path: /