FWIW I recently wrote a Discourse plug-in proposal to help communities migrate their existing content from Google Groups to Discourse, while giving members who prefer email the ability to fully participate from their inbox without any major changes from the current Google Groups experience. I didn't get a ton of interest so I haven't moved forward in productizing this, but all of the email parsing code is already written (and in production), so I can still ship a plug-in if this is something that other folks actually care about:

https://meta.discourse.org/t/proposed-plugin-to-improve-repl...