postfix - What is wrong with this email headers? It always go to gmail and yahoo spam box

Delivered-To: xxxx@gmail.com

Received: by 10.229.249.207 with SMTP id ml15cs152516qcb;
        Tue, 9 Aug 2011 05:22:45 -0700 (PDT)

Received: by 10.151.142.17 with SMTP id u17mr6831320ybn.198.1312892565452;
        Tue, 09 Aug 2011 05:22:45 -0700 (PDT)


Return-Path: 

Received: from mail.mydomain.com.br (mail.mydomain.com.br [xx.xx.xxx.xx])
        by mx.google.com with ESMTP id g4si5211294ybi.22.2011.08.09.05.22.44;
        Tue, 09 Aug 2011 05:22:44 -0700 (PDT)

Received-SPF: pass (google.com: domain of xxxxx@xxxxxxxxx designates xx.xx.xxx.xx as permitted sender) client-ip=x.xx.xxx.xx;

Authentication-Results: mx.google.com; spf=pass (google.com: domain of xxxxx@xxxxxxxxx designates x.xx.xxx.xx as permitted sender) smtp.mail=xxxxx@xxxxxxxxx; dkim=pass header.i=@mydomain.com.br


Received: from mydomain.com.br (xxxxxxxxxxxxxxx.xxxxxxxxxxxxxxx [x.xx.xxx.xx])
    by mail.mydomain.com.br (Postfix) with ESMTPA id 02C12B543AE
    for ; Tue,  9 Aug 2011 09:22:42 -0300 (BRT)

DomainKey-Signature: a=rsa-sha1; s=mail; d=mydomain.com.br; c=simple; q=dns;
    b=K7lPw72zqmctULDiEdOjdgQWpmClkICAjdKo07hkSicyQU7nRjFAQXR9hf270t0aq
    wD5XjP35FRY3a52kNsVo1OAI22ttiKEDUC/N9e0/57cnoTXFVvzumYBSylgKyk3mPbu
    H0cEJtAVlutfgCES8x1Gdjigd/ek11WQabWWjnk=

DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=mydomain.com.br;

    s=mailk; t=1312892562;
    bh=1lADbHhk2GRaz/mXB4uRhcq7+ei8fg20sFxDsiIDW4A=;
    h=Date:To:From:Reply-to:Subject:Message-ID:MIME-Version:
     Content-Type;
    b=NN2JnPJJZfWnAjYU8x1dC5Cwz3WbkwPAx1yU0w9Xbz55Aa2KpbpIYnTiRmbKg7L0O
     7aP0EgAoqCMRwni6S6HqLw3MjR2I3B3qVmkMkyKMHjZ6N2sivovrTq/R0P9A4X+8G9
     GLim31Yo/ZTLl3cEyOrWCvokAVJYMSm/uS8aTpaY=

Date: Tue, 9 Aug 2011 09:22:42 -0300


To: xxxxx@xxxxxxxxx

From: xxxxx 

Reply-to: xxxxx 

Subject: =?iso-8859-1?Q?Reuni=E3o_de_professores_na_escola?=

Message-ID: 


X-Priority: 3

X-Mailer: PHPMailer 5.2 (http://code.google.com/a/apache-extras.org/p/phpmailer/)

MIME-Version: 1.0

Content-Type: multipart/alternative;
    boundary="b1_b8b16385ed9c0141365ee6257a39d284"



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Content-Type: text/plain; charset="iso-8859-1"
Content-Transfer-Encoding: 8bit

Teremos reunião na escola a noite, favor comparecer. obrigado.

--b1_b8b16385ed9c0141365ee6257a39d284
Content-Type: text/html; charset="iso-8859-1"
Content-Transfer-Encoding: 8bit





















Teremos reunião na escola a noite, favor comparecer. obrigado.






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Mail always to Gmail and Yahoo always go to spam.

The server is dedicated and it's not blacklisted.

Perhaps it could be some postfix config?