I've recently finished setting up Exchange 2010 and I think i've configured it correctly but I am unable to access OWA from outside of the network.
If I browse to https://webmail.domain.com/owa I get a page cannot be found error. It works with the IP address and doing an nslookup webmail.domain.com is resolving to the correct IP.
I've also had a DNS entry setup for autodiscover.domain.com and that is working perfectly. If I browse to that I am getting the IIS7 holding page.
I've configured the external URL on the CAS server but i'm wondering if I have missed a step.
First please try to use https://webmail.domain.com/owa from inside the network to verify if we can find the logon page.
Please let me know if you have any firewall in front of the network.
Please let me know how did you publish OWA to internet.
Please check the A record, it should be pointed to the CAS server or router or firewall.