The people who develop discourse have mail systems that work. They aren’t going to add a feature for people who want to use multiple unreliable mail systems. You can develop a plugin if you want. It will be difficult to develop and will be troublesome to maintain.
It may be easy to install postfix, but it’s much harder to run a functioning mail server now than it was when I ported Sendmail and uucp to Linux. If running a mail server were easy you would have a working mail server and not want two.
That’s not what I think, but you might know a lot more about discourse plugin development than I do.