What are the appropriate SMTP headers to not land in inbox for Drupal 6 default contact form

Question

What are the appropriate SMTP headers for scenario below, that are most likely to avoid e-mail from going to SPAM or just be rejected by receiver SMTP service.

Currently I have a Drupal 6 default contact form that sends the e-mail with SMTP header FROM being the email of the person who filled the form. GMAIL is gladly accepting such an e-mail, but hotmail is rejecting it completely and does it deliver it to my client's inbox. I can try to modify the e-mail headers in the email being sent as Drupal API makes this possible. However, I am not sure if hotmail has black listed my server either.

Following are the SMTP headers I receive:

Delivered-To: MYEMAIL@gmail.com
Received: by 10.70.67.69 with SMTP id l5csp52700pdt;
Fri, 28 Mar 2014 07:47:10 -0700 (PDT)
X-Received: by 10.42.156.2 with SMTP id x2mr967266icw.82.1396018029804;
Fri, 28 Mar 2014 07:47:09 -0700 (PDT)
Return-Path: <WEBSITEUSER@gt.rr.com>
Received: from [CLIENT1AND1SUERNAME].onlinehome-server.com ([SERVER_IP_ADDRESS])
by mx.google.com with ESMTPS id m8si6177519igv.4.2014.03.28.07.47.09
for <MYEMAIL@gmail.com>
(version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128);
Fri, 28 Mar 2014 07:47:09 -0700 (PDT)
Received-SPF: softfail (google.com: domain of transitioning [WEBSITEUSER]@gt.rr.com does not designate MY_SERVER_IP_ADDRESS as permitted sender) client-ip=MY_SERVER_IP_ADDRESS;
Authentication-Results: mx.google.com;
spf=softfail (google.com: domain of transitioning [WEBSITEUSER]@gt.rr.com does not designate MY_SERVER_IP_ADDRESS as permitted sender) smtp.mail=[WEBSITEUSER]@gt.rr.com
Received: from www.CLIENTWEBSITE.com (localhost [127.0.0.1])
by [CLIENT1AND1SUERNAME].onlinehome-server.com (Postfix) with ESMTPA id D4633E2C40E;
Fri, 28 Mar 2014 09:46:38 -0500 (CDT)
Date: Fri, 28 Mar 2014 09:46:38 -0500
To: MYCLIENT@hotmail.com, MYEMAIL@gmail.com
From: MY CLIENT COMPANY NAME <WEBSITEUSER@gt.rr.com>
Reply-to: WEBSITEUSER@gt.rr.com
Subject: [Feedback] [USER TYPED SUBJECT]
Message-ID: <MESSAGEID@www.CLIENTWEBSITE.com>
X-Priority: 3
X-Mailer: PHPMailer (phpmailer.codeworxtech.com) [version 2.2]
Errors-To: info@CLIENTWEBSITE.com
Sender: info@CLIENTWEBSITE.com
MIME-Version: 1.0
Content-Transfer-Encoding: 8Bit
Content-Type: text/plain; charset="utf-8"

Answer 1

The issue was caused by comcast blocking outbound/inbound port 25.