The best thing I have found is this software, mailman. Check out an example: https://lists.openstack.org/archives/list/openstack-discuss@lists.openstack.org/thread/GTPTFUPXXBDMWNQZGZDLM2IIX4FSTT5Y/
It lets you view discussions as threads on a website, and then you when click reply, it gives you the option to reply using your email software. This is a really neat way to do it, although the UI definitely looks rough compared to lemmy. But it might be a better way to do email, where you literally just present it via a forum website but all "posts" and "comments" are handled by your email client.
Or mailman lets you sign in and reply via the software itself, but idk if they are actually using that.