During checkout, we're trying to send an html email to an email address. It works, but just not html. I've tested sending a newsletter and I can receive html emails fine from my zen cart install. All my email options settings are set to enable html. The email transport method is PHP.

Here's the code I'm using. If you guys could point me in the right direction it would be much appreciated.

Code:
$subject = "Test Subject";
$body = "<p>The body of the email.</p>";
zen_mail("", $email, $subject, $body, STORE_NAME, STORE_ADMIN_EMAIL, array("EMAIL_MESSAGE_HTML" => $body));