NewBie Question about Mail

we have recently created our Mail service running on Leopard. We can send and received internally but how can we send outside like if we would like to send to a gmail account or hotmail? also we are using a different dns so as not to conflict with the existing one we have. how can we be able t use the existing domain from the mail server. e.g. existing domain = test.com email = user@test.com local mail server = example.com email = we wanted to use same as user@test.com any advice on what to do?