Recently Gmail has started marking emails sent directly from my server (using php mail()) as spam and I'm having problems trying to find the issue. If I send an exact copy of the same email from my email client it goes to the Gmail inbox. The emails are plain text, around 7 lines long and contain a URL link in plain text.

As the emails sent from my client are getting through fine I'm thinking that the content isn't the issue. It would be greatly appreciated if someone could take a look at the the following headers and give me some advice why the email from the server is being marked as spam.

Email from Server:

Delivered-To: [email protected]
Received: by 10.49.98.228 with SMTP id el4csp101784qeb;
    Thu, 15 Nov 2012 14:58:52 -0800 (PST)
Received: by 10.60.27.166 with SMTP id u6mr2296595oeg.86.1353020331940;
    Thu, 15 Nov 2012 14:58:51 -0800 (PST)
Return-Path: >[email protected]>
Received: from dom.domainbrokerage.co.uk (dom.domainbrokerage.co.uk. [174.120.246.138])
    by mx.google.com with ESMTPS id df4si17005013obc.50.2012.11.15.14.58.51
    (version=TLSv1/SSLv3 cipher=OTHER);
    Thu, 15 Nov 2012 14:58:51 -0800 (PST)
Received-SPF: pass (google.com: domain of [email protected] designates 174.120.246.138 as permitted sender) client-ip=174.120.246.138;
Authentication-Results: mx.google.com; spf=pass (google.com: domain of [email protected] designates 174.120.246.138 as permitted sender) [email protected]; dkim=pass [email protected]
DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=domainbrokerage.co.uk; s=default;
    h=Date:Message-Id:Content-Type:Reply-to:From:Subject:To; bh=2RJ9jsEaGcdcgJ1HMJgQG8QNvWevySWXIFRDqdY7EAM=;
    b=mGebBVOkyUhv94ONL3EabXeTgVznsT1VAwPdVvpOGDdjBtN1FabnuFi8sWbf5KEg5BUJ/h8fQ+9/2nrj+jbtoVLvKXI6L53HOXPjl7atCX9e41GkrOTAPw5ZFp+1lDbZ;
Received: from grantw by dom.domainbrokerage.co.uk with local (Exim 4.80)
    (envelope-from >[email protected]>)
    id 1TZ8OZ-0008qC-Gy
    for [email protected]; Thu, 15 Nov 2012 22:58:51 +0000
To: [email protected]
Subject: Offer Accepted
X-PHP-Script: www.domainbrokerage.co.uk/admin.php for 95.172.231.27
From: My Name >[email protected]>
Reply-to: [email protected]
Content-Type: text/plain; charset=Windows-1251
Message-Id: >[email protected]>
Date: Thu, 15 Nov 2012 22:58:51 +0000
X-AntiAbuse: This header was added to track abuse, please include it with any abuse report
X-AntiAbuse: Primary Hostname - dom.domainbrokerage.co.uk
X-AntiAbuse: Original Domain - gmail.com
X-AntiAbuse: Originator/Caller UID/GID - [500 500] / [47 12]
X-AntiAbuse: Sender Address Domain - domainbrokerage.co.uk
X-Get-Message-Sender-Via: dom.domainbrokerage.co.uk: authenticated_id: grantw/from_h

Email from client:

Delivered-To: [email protected]
Received: by 10.49.98.228 with SMTP id el4csp101495qeb;
        Thu, 15 Nov 2012 14:54:49 -0800 (PST)
Received: by 10.182.197.8 with SMTP id iq8mr2351185obc.66.1353020089244;
        Thu, 15 Nov 2012 14:54:49 -0800 (PST)
Return-Path: >[email protected]>
Received: from dom.domainbrokerage.co.uk (dom.domainbrokerage.co.uk. [174.120.246.138])
        by mx.google.com with ESMTPS id ab5si17000486obc.44.2012.11.15.14.54.48
        (version=TLSv1/SSLv3 cipher=OTHER);
        Thu, 15 Nov 2012 14:54:49 -0800 (PST)
Received-SPF: pass (google.com: domain of [email protected] designates 174.120.246.138 as permitted sender) client-ip=174.120.246.138;
Authentication-Results: mx.google.com; spf=pass (google.com: domain of [email protected] designates 174.120.246.138 as permitted sender) [email protected]; dkim=pass [email protected]
DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=domainbrokerage.co.uk; s=default;
    h=Content-Transfer-Encoding:Content-Type:Subject:To:MIME-Version:From:Date:Message-ID; bh=bKNjm+yTFZQ7HUjO3lKPp9HosUBfFxv9+oqV+NuIkdU=;
    b=j0T2XNBuENSFG85QWeRdJ2MUgW2BvGROBNL3zvjwOLoFeyHRU3B4M+lt6m1X+OLHfJJqcoR0+GS9p/TWn4jylKCF13xozAOc6ewZ3/4Xj/YUDXuHkzmCMiNxVcGETD7l;
Received: from w-27.cust-7941.ip.static.uno.uk.net ([95.172.231.27]:1450 helo=[127.0.0.1])
    by dom.domainbrokerage.co.uk with esmtpa (Exim 4.80)
    (envelope-from >[email protected]>)
    id 1TZ8Ke-0001XH-7p
    for [email protected]; Thu, 15 Nov 2012 22:54:48 +0000
Message-ID: >[email protected]>
Date: Thu, 15 Nov 2012 22:54:50 +0000
From: My Name >[email protected]>
User-Agent: Postbox 3.0.6 (Windows/20121031)
MIME-Version: 1.0
To: [email protected]
Subject: Offer Accepted
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
X-AntiAbuse: This header was added to track abuse, please include it with any abuse report
X-AntiAbuse: Primary Hostname - dom.domainbrokerage.co.uk
X-AntiAbuse: Original Domain - gmail.com
X-AntiAbuse: Originator/Caller UID/GID - [47 12] / [47 12]
X-AntiAbuse: Sender Address Domain - domainbrokerage.co.uk
X-Get-Message-Sender-Via: dom.domainbrokerage.co.uk: authenticated_id: [email protected]