Cable Modem setup for email server

Right now I have my cable modem connected to a switch and our
internal network. We just upgraded to 5 static (public) IP
addresses. I’d like to setup an email server and was wondering
the best network configuration for this. Should I put it on our
internal network and open up the needed ports to that server or
should I setup another seperate internal network and point one
of my static IP addresses to this server. My real question is, how
would you configure the cable modem?

I hope I am explainig this well enough to understand.