Thanks for the reply @codinghorror. I’ve had a search around, I think this is the thread you’re referencing
So, OK, I accept that adding References / In-Reply-To to the first post is a hack (although it’s entirely RFC legal - consider what happens when you join a thread halfway through, your client has no history of the prior emails). I can probably drop that part of the patch.
However, I still need to address the larger point in some way - the headers contain IDs that cannot ever have existed in the mail client of any user (except the sender). I see that in the above thread @zogstrip added the patch that moved from using <topic/#id/#post_id> to using the original incoming mail header - but I must be misunderstanding how that’s supposed to work, because it appears to cause the issue outlined in the first post. I think just using line 97 instead of the if/then/else would solve my problem, but I’m sure there’s more to it.
What did I miss?