This improve previous commit by making sure we never consider the thread_id/model values in the In-Reply-To/References header if the host name did not match. Also fixes the tests that were using the 6.1 compatibility mode to post in a mail group thread instead of specifying the right message-id. bzr revid: odo@openerp.com-20140411142429-y0rpkzqbrsabxqsg