...because Google thinks they are in violation of their recommended email sender guidelines, apparently:
The link points to this Google help page. Looking at the email headers, I guess that at least a SPF record is missing (since the mail has Received-SPF: neutral header set), but probably some other things aren't quite as configured as they should be. Here are the headers of that particular email if they're of any use:
Delivered-To: firstname.lastname@example.org Received: by 10.79.114.130 with SMTP id x2csp696916ivf; Sat, 9 Jul 2016 04:17:17 -0700 (PDT) X-Received: by 10.129.40.194 with SMTP id o185mr1325998ywo.45.1468063037871; Sat, 09 Jul 2016 04:17:17 -0700 (PDT) Return-Path: <email@example.com> Received: from brickimedia.org ([2604:180::a9c8:d19d]) by mx.google.com with ESMTPS id s205si782155ywc.402.2016.07.09.04.17.17 for <firstname.lastname@example.org> (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sat, 09 Jul 2016 04:17:17 -0700 (PDT) Received-SPF: neutral (google.com: 2604:180::a9c8:d19d is neither permitted nor denied by best guess record for domain of email@example.com) client-ip=2604:180::a9c8:d19d; Authentication-Results: mx.google.com; spf=neutral (google.com: 2604:180::a9c8:d19d is neither permitted nor denied by best guess record for domain of firstname.lastname@example.org) email@example.com Received: from brickimedia.org (localhost.localdomain [127.0.0.1]) by brickimedia.org (8.14.4/8.14.4/Debian-8) with ESMTP id u69BHHZa019937 for <firstname.lastname@example.org>; Sat, 9 Jul 2016 07:17:17 -0400 Received: (from www-data@localhost) by brickimedia.org (8.14.4/8.14.4/Submit) id u69BHHDg019936; Sat, 9 Jul 2016 07:17:17 -0400 X-Authentication-Warning: brickimedia.org: www-data set sender to brickipedia using -f To: Jack Phoenix <email@example.com> Subject: LegoFan4000 has sent you the Gold Brick gift on Brickipedia! X-PHP-Originating-Script: 0:UserMailer.php From: Brickipedia <firstname.lastname@example.org> Date: Sat, 09 Jul 2016 11:17:17 +0000 Message-ID: <email@example.com> X-Mailer: MediaWiki mailer List-Unsubscribe: <http://en.brickimedia.org/wiki/Special:Preferences> MIME-Version: 1.0 Content-type: text/html; charset=UTF-8 Content-transfer-encoding: 8bit
@lcawte, maybe you have some thoughts on this as you know lots about mail ops?