This Discourse server was installed using Ansible that was written a long time ago and I’m afraid that there is a problem with incoming email that needs fixing before replying to threads by email is going to work.
This is now fixed! 
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This should now be fixed and this post should trigger an email for me to reply to as a test as I have mailing list mode enabled…
Oops there was a Discourse setting that needed changing first, it should now be working…?
This is a test of the reply-by-email feature of Discourse — some people will prefer to enable mailing list mode and only interface with this site via email and this should now be an option 