|The mailserver does indeed need a public IP, or you can port forward from the router to port 25 on your mailserver (a safer option IMO). You can't assign both IPs in the zone configuration file (well, you can but they would be served on a round-robin basis, which wouldn't be any use). You need two separate zones; as they are both the same name that means two DNS servers. This means either two physical machines or two bind daemons on the one machine bound to different IP addresses.|
Is the DNS server to be the autoritative server for your Internet domain? Most domain registrars provide DNS services in which case you would make an entry for your mail server there. That would be much easier (and, again, more secure) than running a DNS server on your lan providing services to the outside world.