the documentation for iRedmail covers your question. I abandoned trying to host locally because my ISP blocks all email related ports.

a bit late to the party here, but I didnt see iRedmail mentioned. been using this to host my own email on a VPS for a little over a year now and its great. for me its worth, you can absolutely make it secure, and its not stupid to run it off a local computer. unfortunately most ISPs make it insanely difficult to host on your home network.