function &_addHtmlPart(&$obj = null)
    {
        $params['content_type'] = 'text/html';
        $params['encoding']     = $this->_build_params['html_encoding'];
        $params['charset']      = $this->_build_params['html_charset'];
        $params['eol']          = $this->_build_params['eol'];

        if (is_object($obj)) {
            $ret = $obj->addSubpart($this->_htmlbody, $params);
        } else {
            $ret = new Mail_mimePart($this->_htmlbody, $params);
        }

        return $ret;
    }