We are integrating GMail using the latest API provided by them as a part of developing email client within our application. We are using PHP client library provided by Googl
Replace your:
$strRawMessage .= 'Content-type: Multipart/Alternative; boundary="' . $boundary . '"' . "\r\n";
with:
$strRawMessage .= 'Content-type: Multipart/Mixed; boundary="' . $boundary . '"' . "\r\n";