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Here are the example headers of an email sent to a Google address.

Delivered-To: *@gmail.com
Received: by 10.14.100.10 with SMTP id y10csp109835eef;
        Tue, 8 May 2012 01:44:02 -0700 (PDT)
Received: by 10.216.214.82 with SMTP id b60mr2383310wep.38.1336466642775;
    Tue, 08 May 2012 01:44:02 -0700 (PDT)
Return-Path: <www@mywebsite.com>
Received: from mywebsite.com (mywebsite.com. [mywebsite-ip])
    by mx.google.com with ESMTP id p5si22005299wel.112.2012.05.08.01.44.02;
    Tue, 08 May 2012 01:44:02 -0700 (PDT)
Received-SPF: pass (google.com: best guess record for domain of www@mywebsite.com designates mywebsite-ip as permitted sender) client-ip=mywebsite-ip;
Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of www@mywebsite.com designates mywebsite-ip as permitted sender) smtp.mail=www@mywebsite.com; dkim=pass header.i=@mywebsite.com
Received: by mywebsite.com (Postfix, from userid 80)
id E8A63322; Tue,  8 May 2012 08:43:52 +0000 (UTC)
DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=mywebsite.com; s=dkim;
t=1336466632; bh=Z7xJudGc3lcz/jf/0hPLSOBFSCA07D0D3oJwgQbIhn4=;
h=To:Subject:Date:From;
b=ZSc4A/aUALtqhVe88Nc5QDaWanKMOZgs3PEed99P42amZEhrp3Q3Qi1F90KJ9MuzK
 ZBzMmvycM8cSXdUno8mJZhjtPU4YEzgP3BetLZImlZC8Z0H84YVc9FGimRyt5LlOVI
 Zb13gvWL4ihO7ABSte29AewocINOtumktA+hZ5UQ=
To: "test" <*@gmail.com>
Subject: Verifica indirizzo email
X-PHP-Script: _/index.php for my-ip
Message-ID: <1336466632.4fa8dcc8e42e8@_>
Date: Tue, 08 May 2012 08:43:52 +0000
From: "mywebsite.com" <accounts@mywebsite.com>
MIME-Version: 1.0
Content-Type: text/html; charset=utf-8
Content-Transfer-Encoding: quoted-printable

Ciao test, grazie per esserti registrato a mywebsite.<br /><br />Segui que=
sto link per validare il tuo indirizzo email:<br /><br />http://mywebsite.=
com/user/validate_email?a=3D1-35ae755914

I send about 20 email/day; no big deal! The IP i send from is not blacklisted. The reverse ip is correct.

DKIM and SPF checks are passed.

What could it be?

share|improve this question
1  
You might have to contact their support for a true answer, but I'm sure its the total amount/quota of emails from that same address per day / month through their servers... Doesn't it depend on the recipient to determine if it is spam or not? – Zero Stack May 8 '12 at 9:17
    
Hi, this comes from a dedicated server that i control (i have the server from a few months to now), so the number of email sent every day is really 20. Maybe a little more, like 30, but this is the order of magnitude. I'm not sending other emails other than the validation ones (e.g., i don't send notifications like Facebook); so the number of email sent isn't the problem. Maybe i should change the simple HTML used to a plain text format, that might help. I was used to send a daily newsletter to 4000 registered users without any problem, so i really can't understand whats wrong. – cedivad May 8 '12 at 9:21
    
(comment too long) i also don't think that their support is willed to help, why should they? they are Google, not me ;) – cedivad May 8 '12 at 9:22
1  
For a start, you are sending invalid HTML (no doctype, no title tag), and -- apparently -- invalid Message-ID, which must have a domain name after the @. – grawity May 8 '12 at 9:35
1  
@cedivad: It will not be displayed, but as I recall, the <title> tag was required in HTML documents, even if it was empty. Okay, it's unlikely that a spamfilter would implement complete HTML validation, but it won't hurt. A basic HTML 5 document would be <!DOCTYPE html> <title></title> <p>Ciao test, grazie... <p>Segui questo... <p>http://mywebsite.com/... – grawity May 8 '12 at 11:26

I'd have a look at http://www.mxtoolbox.com/blacklists.aspx or any other blacklist/SPAM checkers, but if that doesn't show up that you are on any of the blacklists/spamlists the best idea is to contact the receiving admins to ask them why they're marking your mails as SPAM.

share|improve this answer
    
The source IPs are not marked as spam. May i ask you how would you contact Google MSN or Yahoo to ask such a thing? – cedivad May 8 '12 at 12:07

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