[Users] Thread using subject doesn't seem to work

[Paul]

On Tue, 29 Sep 2015 08:52:34 +0200
Francis Moreau <francis.moro at gmail.com> wrote: 

> I have a folder which contains bug reports sent by bugzilla.
> Unfortunately comments from the same report aren't showed as thread.

Bugzilla messsages use MessageID and References headers, so they
don't need this option to thread properly.

with regards

Paul