./discourse-setup is for people with limited (or no) sysadmin skills. If you’re able to configure a mail server to accept mail from a given IP, then you’re able to use vim and edit the mail config. Surely, you did that to configure the mail server. 
Ah! That I can help with! There’s an undocumented feature. discourse-setup sleeps for 5 seconds after it writes the config file. Just run the wizard and ^C a second or two after you’ve hit enter after the last prompt.
You’ve made the point that I’m not the only person who might make use of that feature. Next time I submit a PR I’ll add a prompt that mentions that delay.