I recently pushed a fix that will:
-
Only set the “
In-Reply-To” and “References” headers when it’s a reply (ie.post_number > 1) - Set the “
In-Reply-To” header to the newest of the referenced (either replied to or quoted) posts - Set the “
References” header to the topic as well as all referenced posts (if any).
This should ensure the emails are threaded properly on any decent 
client 