Virtual domain mail to Internet ?

[Arif TelkomNet]

Dear all, I have been installed vpopmail with qmail and have 3(three) virtual domain in the local area network, with private IP (192.168.0.1). In this server I have  installed DNS too. How to setting connection my mail server to my ISP (Internet Service Provider), if I have both registered IP address and domain so that my 'all virtual domain'  can deliver and receive from/to other domain registered in the Internet ? I hope any suggested  for this solution.   Thanks Arif