I consider it a pretty terrible solution to your problem, but someone asked for it and I wrote GitHub - pfaffman/discourse-email-include-address: Include email addresses on emailed notifications · GitHub.
It adds the sender email address to the emails, so people can reply directly if they want. This exposes everyone’s email address, so it’s really appropriate only for people who would rather be using Mailman than Discourse.
It’s likely that the client I wrote it for hasn’t upgraded since November 2020, so I don’t know whether it still works. At first glance, it would appear that it does.