I've a Postfix installation which works fine, I don't get any trouble with mails sent through a mail client (in my case, Thunderbird or RoundCube) when the To: address is a GMail account. However, I recently needed to use the PHPMailer tool to send some e-mails to some GMail accounts, so I configured an account to be used via SASL authentication + TLS. I don't mean mass mailing, just 2-3 mails. If I send the e-mail from the Thunderbird or RoundCube clients, the mail is not marked as spam. However, if I use PHPMailer, it always gets catalogued as spam. So I compared both headers and I just can't find the reason why the second is marked as spam while the first one is just ok. The first header sent from a mail client which is not marked as spam: Delivered-To: [email protected] Received: by 10.76.153.102 with SMTP id vf6csp230573oab; Tue, 19 Aug 2014 11:08:19 -0700 (PDT) X-Received: by 10.60.23.39 with SMTP id j7mr45544050oef.20.1408471699715; Tue, 19 Aug 2014 11:08:19 -0700 (PDT) Return-Path: <[email protected]> Received: from mail.mydomain.com (X.ip-92-222-X.eu. [92.222.X.X]) by mx.google.com with ESMTPS id t5si27115082oej.10.2014.08.19.11.08.18 for <[email protected]> (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 19 Aug 2014 11:08:19 -0700 (PDT) Received-SPF: pass (google.com: domain of [email protected] designates 92.222.X.X as permitted sender) client-ip=92.222.X.X; Authentication-Results: mx.google.com; spf=pass (google.com: domain of [email protected] designates 92.222.X.X as permitted sender) [email protected]; dkim=pass (test mode) [email protected] Received: by mail.mydomain.com (Postfix, from userid 111) id D8F69120293D; Tue, 19 Aug 2014 19:08:17 +0100 (BST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=mydomain.com; s=mail; t=1408471697; bh=wKMX9gkQ7tCLv8ezrG5t4bICm/SSLQsNfTdZMToksWw=; h=Date:From:To:Subject:From; b=qRNcYVdmk+n3D1uuv0FInTx7/LzH2ojck9DgCmabFPvfke233lkojUOjezCUGx7iV DL8EayZ28mzzzHpB7ETeMzop/5OS3BmvFtGKVD9gzc78cDIFXTDoRFAnkRWDR2IOxI SOn5tiyODTFpkbDgJOndzQ6qL5K0S9ASNGCZrNL4= X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on vpsX.ovh.net X-Spam-Level: X-Spam-Status: No, score=-1.0 required=3.0 tests=ALL_TRUSTED,T_DKIM_INVALID autolearn=ham autolearn_force=no version=3.4.0 Received: from [192.168.1.111] (unknown [77.231.X.X]) (using TLSv1 with cipher ECDHE-RSA-AES128-SHA (128/128 bits)) (No client certificate requested) (Authenticated sender: [email protected]) by mail.mydomain.com (Postfix) with ESMTPSA id 910341202624 for <[email protected]>; Tue, 19 Aug 2014 19:08:17 +0100 (BST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=mydomain.com; s=mail; t=1408471697; bh=wKMX9gkQ7tCLv8ezrG5t4bICm/SSLQsNfTdZMToksWw=; h=Date:From:To:Subject:From; b=qRNcYVdmk+n3D1uuv0FInTx7/LzH2ojck9DgCmabFPvfke233lkojUOjezCUGx7iV DL8EayZ28mzzzHpB7ETeMzop/5OS3BmvFtGKVD9gzc78cDIFXTDoRFAnkRWDR2IOxI SOn5tiyODTFpkbDgJOndzQ6qL5K0S9ASNGCZrNL4= Message-ID: <[email protected]> Date: Tue, 19 Aug 2014 19:08:24 +0100 From: My Name <[email protected]> User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:24.0) Gecko/20100101 Thunderbird/24.6.0 MIME-Version: 1.0 To: My other account <[email protected]> Subject: . Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit . The second header sent from PHPMailer which is always marked as spam: Delivered-To: [email protected] Received: by 10.76.153.102 with SMTP id vf6csp230832oab; Tue, 19 Aug 2014 11:12:10 -0700 (PDT) X-Received: by 10.60.121.67 with SMTP id li3mr44086252oeb.17.1408471930520; Tue, 19 Aug 2014 11:12:10 -0700 (PDT) Return-Path: <[email protected]> Received: from mail.mydomain.com (X.ip-92-222-X.eu. [92.222.X.X]) by mx.google.com with ESMTPS id w8si27103806obn.30.2014.08.19.11.12.10 for <[email protected]> (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 19 Aug 2014 11:12:10 -0700 (PDT) Received-SPF: pass (google.com: domain of [email protected] designates 92.222.X.X as permitted sender) client-ip=92.222.X.X; Authentication-Results: mx.google.com; spf=pass (google.com: domain of [email protected] designates 92.222.X.X as permitted sender) [email protected]; dkim=pass (test mode) [email protected] Received: by mail.mydomain.com (Postfix, from userid 111) id 1999D120293D; Tue, 19 Aug 2014 19:12:09 +0100 (BST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=mydomain.com; s=mail; t=1408471929; bh=N1JuHq1S+8GrjHcEK3xn8P1JS+ygEBv5LKe0BiXuVJo=; h=Date:To:From:Reply-to:Subject:From; b=K7tcPyArzSTY91VEw6mAAFtDurSGwgTLGkfUZdC5mqsg0g/1LzmZkgwdjj4NdJa6M E2kDz3dwYN8FcZmbampJYFXxj4NQVtSnzjiWV40rpfOFqD2rXDGNIyB2QOjBZZ4WK3 7s4lyoJ/BrdQH4en8ctLVsDHed/KpHD4iGFEl67E= X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on vpsX.ovh.net X-Spam-Level: X-Spam-Status: No, score=-1.0 required=3.0 tests=ALL_TRUSTED,T_DKIM_INVALID autolearn=ham autolearn_force=no version=3.4.0 Received: from rpi.mydomain.com (unknown [77.231.X.X]) (using TLSv1 with cipher ECDHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) (Authenticated sender: [email protected]) by mail.mydomain.com (Postfix) with ESMTPSA id B42AF1202624 for <[email protected]>; Tue, 19 Aug 2014 19:12:08 +0100 (BST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=mydomain.com; s=mail; t=1408471928; bh=N1JuHq1S+8GrjHcEK3xn8P1JS+ygEBv5LKe0BiXuVJo=; h=Date:To:From:Reply-to:Subject:From; b=iXPM0tS36swudPTT4FOHHtPi5Ll6LbR60kNqCinZ8utcWoFE31SFTpoMEq5aCM5ux wQMdFiN8c6vkjRGabmvqFTTIbwJsrToHo/4+Lt5HEBoQQE2Y3T+xGmnmGAHCS6stKB yb7SVmtrIAsVtSMKA8VYIbmu2oYqV3afYt7g0OMQ= Date: Tue, 19 Aug 2014 20:12:07 +0200 To: [email protected] From: Trying another account <[email protected]> Reply-to: Trying another account <[email protected]> Subject: . Message-ID: <[email protected]> X-Priority: 3 X-Mailer: PHPMailer 5.1 (phpmailer.sourceforge.net) MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset="UTF-8" . I also tried: Adding a User-Agent header to match the first one. Removing the X-Mailer header. No one of them made a difference. Is there some significant difference which is making the second e-mail to be marked as spam by Google?