private function renderPlainText(&$content, $text, $encoding, $boundary)
    {
        $content .= "\r\n";
        $content .= "--$boundary\r\n";
        $content .= "Content-Type: text/plain;\r\n\tcharset=\"$encoding\"\r\n";
        $content .= "Content-Transfer-Encoding: quoted-printable\r\n";
        $content .= "\r\n";
        $content .= "$text\r\n";
    }