PHP Mail getting blocked from our Exchange Server

Hopefully this will be a easy one someone can help me with.

System:
MS Windows Server 2003 R2
Enterprise x64 Edition SP2
Exchange 2007 Standard Edition.


Awhile back, the company I work for, hired a third party vendor to complely redesign our website.  They set up a contact page, and on this contact page is a form to fill out to ask whatever questions the customer may want to submit.  When the customer hits submit, it sends all the information to Sales@domain.com.  I set up a distribution address on our Exchange server called Sales@domain.com and added a few users who would be answering those emails.

For testing purposes I added my name to the distribution list (sales@domain.com). The problem is, when I send a message using the contact form on the website, the information never makes it to sales@domain.com.  I've searched through the message tracking logs on exchange and I do not see anywhere in there that it made it to the exchange server. I also checked our GFI Mail Security Suite and there is no trace of it ever getting caught up there either.  I know that sales@domain.com works because I can send a email from a personal gmail account and everyone on the distribution list receives it.  I also know that the contact form works as well.  I had the website developer add my personal gmail account, and yahoo account and I received it at both accounts.  So the problem lies on my side of the network.  

I emailed the website guys to ask them how the webserver was sending the mail and this is the response I received back.

We're using PHP Mail() function http://php.net/manual/en/function.mail.php

$response = mail(implode(',',$to_address), $email_subject, $email_message, $headers);

Since it seems to work at any account outside of our exchange accounts, here is what I get to my gmail account when I go to "show original" inside the message.

Delivered-To: user@gmail.com
Received: by 10.220.39.138 with SMTP id g10csp161106vce;
        Thu, 15 Mar 2012 05:11:32 -0700 (PDT)
Received: by 10.50.153.165 with SMTP id vh5mr16567807igb.4.1331813492137;
        Thu, 15 Mar 2012 05:11:32 -0700 (PDT)
Return-Path: <root@u12345678.webserver.com>
Received: from u12345678.webserver.com (u12345678.webserver.com. [12.123.123.101])
        by mx.google.com with ESMTPS id d20si1730483igv.73.2012.03.15.05.11.31
        (version=TLSv1/SSLv3 cipher=OTHER);
        Thu, 15 Mar 2012 05:11:32 -0700 (PDT)
Received-SPF: pass (google.com: best guess record for domain of root@u12345678.webserver.com designates 12.123.123.101 as permitted sender) client-ip=12.123.123.101;
Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of root@u12345678.webserver.com designates 12.123.123.101 as permitted sender) smtp.mail=root@u12345678.webserver.com
Received: (qmail 30110 invoked by uid 48); 15 Mar 2012 08:21:41 -0400
Date: 15 Mar 2012 08:21:41 -0400
Message-ID: <20120315122141.30108.qmail@u12345678.webserver.com>
To: sales@domain.com, user@gmail.com
Subject: Domain.com Contact Us Submission
From: Company Website <company_website@domain.com>
Content-Type: text/html;charset="iso-8859-1"

name: blah<br>
email: user@gmail.com<br>
telephone: 5555555555<br>
comment: testing the form<br>

Anyone have any ideas on things I can look at to see why this is never hitting our exchange server?

Thanks!!

-Zachary