It looks as though multipart messages aren’t being parsed correctly. I made a quick fix to wp-mail.php:
Approx 165 lines in, you’ll see:
if ($content_type == ‘multipart/alternative’) {
$content = explode(‘–‘.$boundary, $content);
$content = $content[2];
$content = explode(‘Content-Transfer-Encoding: quoted-printable’, $content);
$content = strip_tags($content[1], ‘<img><p><br><i><b><u><strike><font><span><div>’);
}
Just change the line that reads:
$content = explode(‘Content-Transfer-Encoding: quoted-printable’, $content);
into:
$content = explode(“\r\n\r\n”, $content, 2);