Is this something to do with SPF failures?
Received-SPF: fail (google.com: domain of
ianb@xx.com does not designate 23.27.50.10 as permitted sender) client-ip=23.27.50.10;
Authentication-Results: mx.google.com; spf=fail (google.com: domain of
ianb@xx.com does not designate 23.27.50.10 as permitted sender) smtp.mailfrom=ianb@xx.com; dkim=pass header.i=@xx.com
Received: from mail.xx.com (mail.xx.com [xx])
(using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client
certificate requested) by mailfwd1.zoneedit.com (Postfix) with ESMTPS id
2F8E02787689 for <ian@x.co.uk>; Fri, 18 Mar 2016 14:06:08 -0400 (EDT)
Received: from localhost (localhost [127.0.0.1]) by mail.xx.com
(Postfix) with ESMTP id 05081B400CF for <ian@xx.co.uk>; Fri, 18 Mar 2016
18:06:02 +0000 (GMT)