XML E-mail to ADF Suddenly Stopped Working

Oh, and in case nobody knows what ADF is...

Here is the Wikipedia entry on it, and here is the PDF documentation.

ADF is basically the universal format for customer/lead data for auto dealerships around the world. It is how they communicate between all the various departments, including service, sales, distribution, etc.

If it helps, here is the XML output that I am sending (in text-only email format). I will paste example email headers below the code:

<?XML VERSION=“1.0”?>
<?ADF VERSION="1.0"?>
<adf>
    <prospect>
        <requestdate>2012-11-30</requestdate>
        <vehicle>
            <year>2013</year>
            <make>Audi</make>
            <model>A4 2.0T</model>
        </vehicle>
        <customer>
            <contact>
                <name part="full">TESTING ADF LEAD FORM – XML STRUCTURE REVISED</name>
                <phone>(123)456-7890</phone>
                <email>someonewho@isntme.com</email>
            </contact>
                <comments>Not Specified</comments>
        </customer>
        <vendor>
            <contact>
                <name part="full">VENDOR NAME</name>
                <phone>123-456-7890</email>
                <email>test@autowebsite.com</phone>
            </contact>
        </vendor>
        <provider>
            <name part="full">PROVIDER NAME</name>
            <service>SERVICE</service>
            <url>http://somewhere.com/</url>
            <email>test@somewhere.com</email>
            <phone>123-456-7890</phone>
        </provider>
    </prospect>
</adf>

Email headers (note the OMITTED elements were omitted by me, of course):

X-Priority: 3
X-Mailer: PHPMailer 5.2.1 (http://code.google.com/a/apache-extras.org/p/phpmailer/)
MIME-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
X-SMTPCOM-Tracking-Number: OMITTED
X-SMTPCOM-Sender-ID: OMITTED
X-SMTPCOM-Spam-Policy: SMTP.com is a paid relay service. We do not tolerate UCE of any kind. Please report it ASAP to abuse@smtp.com
Content-Transfer-Encoding: quoted-printable
Return-Path: OMITTED
X-MS-Exchange-Organization-SCL: 1
X-MS-Exchange-Organization-AVStamp-Mailbox: OMITTED
X-MS-Exchange-Organization-AuthSource:
 OMITTED
X-MS-Exchange-Organization-AuthAs: OMITTED

I'm thinking that the issue may have to do with the content-type and charset I was using to send. Perhaps an auto-upgrade to the CRM is now disallowing the content-type text/plain (which I now realize is PROBABLY wrong). In your experience, when sending XML content via email to be parsed, what is the correct content-type and charset?

My research has come up with:

Content-Type: Application/xml-external-parsed-entity
Charset: UTF-8

Would this be correct?

Still nothing, unfortunately. Can you determine any other reason why a recent update would have affected the format or headers of the system which was previously working with text/plain as content-type? Tried the above, as well as text/xml content types and nothing has worked.