public function setType($type)
    {
        $allowed = array(
            Zend_Mime::MULTIPART_ALTERNATIVE,
            Zend_Mime::MULTIPART_MIXED,
            Zend_Mime::MULTIPART_RELATED,
        );
        if (!in_array($type, $allowed)) {
            /**
             * @see Zend_Mail_Exception
             */
            // require_once 'Zend/Mail/Exception.php';
            throw new Zend_Mail_Exception('Invalid content type "' . $type . '"');
        }

        $this->_type = $type;
        return $this;
    }