Got it, that’s helpful … and I was being dense
connected via telnet to port 25, ran the regular debug script:
RCPT to: nobody@discourse.artisans-collab.org … etc
This worked fine, log says “delivered” so I’ll go back to the config and see if I can get reply-by-email working now 
p.s.
I’m sure somewhere continuing to debug was DNS …
Because I kept sending test messages from my regular email address (a large Uni) and apparently changes to DNS haven’t managed to propagate in to said large email infrastructre.
So I sent a test message from that openinbox account I was going to use for POP3, bingo, delivered fine
god I used to run dns servers, but it was a couple decades ago, and times were simpler.
Anyway, my MX (namecheap) looks exactly as you said, … I’ll stop obfuscating
