Continuing the discussion from Danyl Strype - ethusiastic Disintermediator:
There are several issues here. First, full reply-by-email isn’t a default, though it is supported. There are some complex separate issues about the email vs forum ways of working (such as how things can be moved around or changed or flagged on a forum in ways that email lists can’t do), and that makes it messy to think of the forum as somehow also full email lists.
Anyway, I know from participation in a different Discourse instance that setting up reply-by-email isn’t as trivial as turning on a setting. I don’t recall the details.
If there’s enough demand for it, we might look into this after we finish our systems migration to OSUOSL and have everything integrated. Right now, email from Discourse is still being sent through our old badly-maintained self-run mail server even though our regular email accounts for the team are running through OSUOSL.