Do you have SSL enabled for OWA?
Have you run the IIS lockdown tool on the OWA server?
Are you certain all of your patches are up to date?
That is what needs to be done to prepare the server for opening the port.
Opening the port depends on your configuration, conduits are the old but still supported method of opening a port, access control lists are the current and future method - both require statics to have been created.
You can post your config if you'd like, or email it to me. You will want to remove any password lines.
Matt, resident pix and MCSE jockey.