@waugh if that toggle is off, then it should ... fall back to postfix automatically. Is postfix not receiving messages for outgoing mail?
When I first installed NodeBB and had not even become aware of the settings, and so obviously had not changed them, and I posted a reply that should have generated e-mail, NodeBB complained that it couldn't open a certain port. I have learned that the port number in question is generally used for SMTP to an MSA, message submission agent.
After I installed Postfix, the same thing happened. But I believe I understand why. Postfix out of the box accepts mail from "mail" via a "sendmail" program that Postfix includes for backward compatibility with Sendmail. I believe that "mail" invokes this "sendmail" and pipes the message to it. This succeeds on my system -- I can send and receive e-mail across the Internet, including to gmail and including to the (paid) disposable-email-address provider that I use. But Postfix out of the box does not have its MSA turned on. This is called "submission" in the "master.cf" configuration file. I have since tried turning that on. I think I need to study and change some of the arguments I am not passing to it, but that I see mentioned in an example someone posted. So what is currently happening, with my MSA turned on but not properly configured, is that NodeBB is showing an error message evidently returned by Postfix, to the effect that "localhost." (note the period at the end) is not one of the domains mentioned in my TLS cert (which it isn't, shouldn't be, and can't be). Since NodeBB knows nothing about TLS (does not prompt for any configuration for it), I take this error message as evidence that NodeBB is now talking to Postfix and Postfix is taking the basic syntax but reporting back on a problem that is happening internally to Postfix between its modules on account of my not having configured them correctly yet.