We use Mac OSX 10.8 with Server 2.2.1.
When creating a calendar appointment on a client connected to the OSX server from within our local network (behind our firewall) we can select calendar resources and view other's availability. Outside our network, and in front of our firewall, this function does not work.
We currently have ports 389 and 636 (LDAP/SSL) as well as 8008 and 8443 (Calendaring/SSL).
My gut is telling me that Bonjour might be helping out when inside our network.
