Usually server listens to port 30999. To find the correct port, open server_settings.xml and try to locate . Or simply run server setting editor to see that.
If you plan to use sd_event, you also need to open the HTTP port, assuming that it is activated. Usually it's the port 30980
Also if you enabled inbound e-mail support you must also allow access to port 25. This is not per client setting, port 25 of the application server must be globally open, or at least open to the people that you want to use inbound e-mail.
Furthermore, if you plan to use agent, then the server must be able to access the port 50998 of the machine where agent is running. Also, if you want "tray notifications" to be working, server must have access to a range of ports, normally from 40999 to 41025 of the client.