Howdy all,
I think I’ve sorted out the problem with reply-to via email for the forum, so you can probably respond to forum posts directly via email, as long as you’re responding from the exact email address you used to sign up (and as long as you or your mail client don’t munge the reply-to address which has a magic key unique to every email notification).
The problem was that the Discourse mail-receiver container had the old ca-certificate bundle, and though I’d already done a rebuild of that container, it actually needed me to manually pull it to get the latest version, for reasons I don’t understand. Sorry it took so long to sort out. I didn’t even notice it was broken for quite a while (I don’t use email responses, but I know many folks do).
Cheers,
Joe