Not sure what Discourse has to do with whether the system may additionally hosts an email server or not? Aside of that fact that this theoretically opens another way to send out emails, of course.
For receiving emails (other topic) however it has an effect, since one cannot host the email receiver container then, at least not to listen on port 25 directly. But using the email receiver API of Discourse directly turned out to be just a 2-3 lines in the Postfix config: Is there a way to only IMAP polling for incoming emails - #2 by MichaIng
But I agree, setting up the email server properly was not exactly an easy task, as mentioned above. But super interesting to learn 
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