Exchange Hosting 2010 Outlook connection problem

Hi.

We have just deployed na Exchange 2010 Hosting solution.

We having difficulties configuring Outlook clients from the outside. We NAT done all NATs and there is connectivity on ports 443, 587, 25, etc. The error: The action cannot be completed. The connection to Microsoft Exchange is unavailable.

From inside everything works. From outside we have OWA only.

Any ideas?

Thanks