I’m not sure if I explained myself very well as I can’t understand why Discourse would put a post in the wrong category. It’s as if it isn’t looking at the route in (which should have priority) but only the message-id. For a single e-mail sent to two mailing lists at the same time there will be one message-id but with two distribution lists (which are independent), two 'To: ’ addresses and two 'Reply-To: ’ addresses (with a different one shown on each of the messages depending on which list it was sent to).
If this is what’s happening then what would happen if someone moved a topic out of a category that was a mailing list mirror, and put it somewhere else on discourse, but then replies (via the mailing list) kept coming in? Would discourse add those replies to the category associated with the mailing list (even though the original post may have been moved away) or would all the replies magically end up in the category that the topic had been moved to?
Can Discourse cope with having two messages in two categories at the same that have the same message-id (but different 'To: ’ addresses?