Spam Filters - Why does the same message fail via one smtp server but not the other?

Question

I have a PHPMailer script sending email. If sent via postfix localhost it is marked as spam with these headers.

Delivered-To: xxxx
Received: by 10.58.210.166 with SMTP id mv6csp82473vec;
        Sun, 15 Sep 2013 16:05:58 -0700 (PDT)
X-Received: by 10.14.184.3 with SMTP id r3mr28558eem.49.1379286358370;
        Sun, 15 Sep 2013 16:05:58 -0700 (PDT)
Return-Path: <editor@xxxx.co.uk>
Received: from server1.xxx.co.uk (server1.xxx.co.uk.       [146.185.134.154])
    by mx.google.com with ESMTP id o7si13323028eep.228.1969.12.31.16.00.00;
    Sun, 15 Sep 2013 16:05:58 -0700 (PDT)
Received-SPF: pass (google.com: domain of editor@xxx.co.uk designates    146.185.134.154 as permitted sender) client-ip=146.185.134.154;
Authentication-Results: mx.google.com;
       spf=pass (google.com: domain of editor@xxx.co.uk designates     146.185.134.154 as permitted sender) smtp.mail=editor@xxx.co.uk
Received: from www.xxx.co.uk (localhost [127.0.0.1])
by server1.xxxx.co.uk (Postfix) with ESMTP id 9F27E609DE
for <xxxx>; Mon, 16 Sep 2013 00:05:57 +0100 (BST)
Date: Mon, 16 Sep 2013 00:05:57 +0100
To: Chris Bridges <xxxx>
From: KCTL Editor <editor@xxx.co.uk>
Reply-To: KCTL Editor <editor@xxx.co.uk>
Subject: Hello
Message-ID: <d54ee978d231d058221bfdca8ae603f0@www.xxx.co.uk>
X-Priority: 3
X-Mailer: PHPMailer 5.2.7 (https://github.com/PHPMailer/PHPMailer/)
MIME-Version: 1.0
Content-Type: text/html; charset=iso-8859-1
Content-Transfer-Encoding: 8bit

if sent via outlook.com smtp server i get these headers, and the message is not marked as spam.

Delivered-To: xxxx
Received: by 10.58.210.166 with SMTP id mv6csp82948vec;
    Sun, 15 Sep 2013 16:19:34 -0700 (PDT)
X-Received: by 10.224.111.200 with SMTP id t8mr10893406qap.21.1379287174535;
    Sun, 15 Sep 2013 16:19:34 -0700 (PDT)
Return-Path: <editor@xxx.co.uk>
Received: from blu0-omc4-s32.blu0.hotmail.com (blu0-omc4-s32.blu0.hotmail.com. [65.55.111.171])
        by mx.google.com with ESMTP id f3si6339439qci.79.1969.12.31.16.00.00;
        Sun, 15 Sep 2013 16:19:34 -0700 (PDT)
Received-SPF: pass (google.com: domain of editor@xxxx.co.uk designates 65.55.111.171 as permitted sender) client-ip=65.55.111.171;
Authentication-Results: mx.google.com;
   spf=pass (google.com: domain of editor@xxxx.co.uk designates     65.55.111.171 as permitted sender) smtp.mail=editor@xxx.co.uk
Received: from BLU0-SMTP127 ([65.55.111.135]) by blu0-omc4-s32.blu0.hotmail.com with    Microsoft SMTPSVC(6.0.3790.4675);
 Sun, 15 Sep 2013 16:19:33 -0700
X-TMN: [Xddl8O20Uhw7uyslAavFGmwTa6R4yhFXBk6Rx2dEr4U=]
X-Originating-Email: [editor@xxx.co.uk]
Message-ID: <BLU0-SMTP127EC145C16ABDF13FF3A2B99250@phx.gbl>
Return-Path: editor@xxx.co.uk
Received: from www.xxx.co.uk ([146.185.134.154]) by BLU0-SMTP127.phx.gbl   over TLS secured channel with Microsoft SMTPSVC(6.0.3790.4675);
 Sun, 15 Sep 2013 16:19:33 -0700
Date: Mon, 16 Sep 2013 00:19:31 +0100
To: Chris Bridges <xxxx>
From: KCTL Editor <editor@xxx.co.uk>
Reply-To: KCTL Editor <editor@xxx.co.uk>
Subject: Hello
X-Priority: 3
X-Mailer: PHPMailer 5.2.7 (https://github.com/PHPMailer/PHPMailer/)
MIME-Version: 1.0
Content-Type: text/html; charset="iso-8859-1"
Content-Transfer-Encoding: 8bit
X-OriginalArrivalTime: 15 Sep 2013 23:19:33.0429 (UTC) FILETIME=[09662650:01CEB26A]

Any ideas on what the different causing this is? Tested with both gmail and outlook.com recipient email addresses with the same outcome. I am completely lost. Most things seem to be fairly similar and SPF passes either way. I am completely stumped!

I am aware there are differences but I see no significant differences?

EDIT

I have some updates. I have now tried adding DKIM signing with no effect. I have also tried telnetting the email so to avoid PHP. Therefore I can only assume the problem is with postfix. The IP address is not blacklisted.

Google gives a very vague reason " It's similar to messages that were detected by our spam filters"

However I do not see how it meets the criteria listed here for this reason: "https://support.google.com/mail/answer/1366858?hl=en&ctx=mail&expand=5

Any input will be greatly appreciated. This is really bugging me now as it is going to delay my launch date.

The email also passes other spam tests with flying colours (e.g. port25.com 's verifier)

==========================================================
Summary of Results
==========================================================
SPF check:          pass
DomainKeys check:   neutral
DKIM check:         pass
Sender-ID check:    pass
SpamAssassin check: ham

Result:         ham  (-4.2 points, 5.0 required)

 pts rule name              description
---- ---------------------- --------------------------------------------------
-2.4 RP_MATCHES_RCVD        Envelope sender domain matches handover relay domain
-1.9 BAYES_00               BODY: Bayes spam probability is 0 to 1%
                        [score: 0.0000]
 0.0 HTML_MESSAGE           BODY: HTML included in message
 0.1 DKIM_SIGNED            Message has a DKIM or DK signature, not necessarily valid

Answer 1

It appears that Google trusts mail arriving from Hotmail more than it trusts mail arriving from some random server on the Internet. This is completely out of your control.

One thing that is in your control is:

X-Mailer: PHPMailer 5.2.7 (https://github.com/PHPMailer/PHPMailer/)

Considering how often this software is abused, I am not surprised to see anyone regard mail containing this as suspicious. I would remove it.

You should also consider adding the RFC 2369 mail headers to your list traffic. Though I doubt that particular piece of software is capable of it. If you're intending to run a mailing list, consider using software designed for that purpose instead.