Email Messages are not threaded in Outlook

The email notifications for Jira/Stash aren't threaded properly in Outlook. Phabricator does this properly, and from the looks of their source, it looks like it might be that you need to set a 'Thread-Index' tag.

It seems like on top of that, there's also a "Thread-Topic" which dictates the subject shown for the thread as a whole, which might be useful if it was set to an Issue Number or something.