I don’t know them myself, and can’t find anything with a quick Google. But, comparing to GitHub notification emails (which are correctly threaded), here are some differences I notice:
- Discourse emails have different Sender headers for each email, whereas GitHub emails do not have a Sender header. (This manifests in the UI via Outlook’s “$Sender on behalf of $From” for Discourse, whereas GitHub just uses “$From”)
- GitHub emails have List-Post headers; Discourse emails do not.
- Discourse emails have different Reply-To addresses; GitHub emails use the same one for each thread. (Both use different Reply-To names.)
I put together a gist with the headers of two Discourse emails in the same thread, plus two GitHub emails in the same thread, which can be analyzed further. I’ll try tweeting to see if anyone can connect me with the Outlook team as well for a more definitive answer.