Blocking all outgoing email, save one domain

I’m setting up mail-in-a-box on my local network in order to provide developers with a way to test email functionality within their applications without fear of emails making it to the outside world (when they’re testing with real data). I’d like to block ALL outgoing email, save one domain (ours).

From what I’ve researched, the best way to do this would be to use Postfix to block all but one (or multiple) domain(s). I’ve followed the instructions that can be found here and everything seems to be working correctly.

Is this the correct way to go about doing this? Would a future update of either Postfix or Mail-in-a-box overwrite the changes I’ve made?

Sorry in advance if this is a silly question and thanks for the help!

Not currently, but anything is possible in future versions.