Can' add Exchange account to Outlook 2013

Hi,

We have an Exchange 2013 server which as far as I know is fully working - Outlook clients on the domain connect fine, OWA works fine and mobile devices work fine.

However I have a problem when trying to add an Exchange account to Outlook 2013 when this computer is on another domain (separate network, separate domain, separate company).

I assume Outlook is searching for an Exchange server which isn't there but how can I get around this?

Thanks