There are many doors to your right.
There is a twisty path to your left.
You are carrying:
Server
Operating system
Choice of MTAs
Changing network
Changing DNS environment
Changing destination email packages with varying levels of authentication requirements.
Firewalls.
And lots of other baggage.
You need to realize the email is a coopoerative endeavor between your system and the system it needs to talk to. So if you are not using a smarthost to hand the email off, so need to make sure your system name and DNS is setup. But if you are behind a firewall, then it is unlikely you have a public facing system, which in turn means you need to deal with address mapping and port level redirection. This quickly escalates into a brain siezure for people doing it the 1st time. And even if you get it to work, it might only be with SOME systems you are talking to, which means troubleshooting is really painful.
So, I think you need allocate the final IP / DNS entries before you go to your next step since that drives much of the configuration.