I believe you're looking for the configuration option to set the external client URL that outlook or activesync devices use when accessing mail either through EWS or activesync.
To set this in exchange 2010 via the EMC (Exchange Management Console):
- Open the EMC.
- Expand the "Server Configuration" tree in the left hand pane.
- Select the "Client Access" item under "Server Configuration"
- In the Actions pane, located to the right of the window, now select "Configure External Client Access Domain" and follow the wizard:
I would also suggest creating an A record in your external and internal DNS servers pointing to your exchange server, this will allow for far easier configuration of your email clients.