Strip the first hop from the email headers

Currently email headers contain the Docker container IP address, for example:

Received: from localhost.localdomain (unknown [172.17.0.2])                                                                                                                       
        by discourse.example.org (Postfix) with ESMTPS id 5H52AC9371                                                                                                       
        for <example@example.org>; Mon, 24 Apr 2017 19:02:00 +0000 (UTC)  

It would be good to remove this.