maybe a little late but i the file

functions.inc.php on line 2361 to 2366

$headers .= "MIME-Version: 1.0" . $mail_header_separator;
$headers .= "X-Sender-IP: ". $_SERVER['REMOTE_ADDR'] . $mail_header_separator;
$headers .= "Content-Type: text/plain; charset=" . $mail_charset . $mail_header_separator;
$headers .= "Content-Transfer-Encoding: quoted-printable";

change your Content type to:
"Content-Type: text/html"
"MIME-Version: 1.0"