public function setMessageId($id = true)
    {
        if ($id === null || $id === false) {
            return $this;
        } elseif ($id === true) {
            $id = $this->createMessageId();
        }

        if ($this->_messageId === null) {
            $id = $this->_filterOther($id);
            $this->_messageId = $id;
            $this->_storeHeader('Message-Id', '<' . $this->_messageId . '>');
        } else {
            /**
             * @see Zend_Mail_Exception
             */
            // require_once 'Zend/Mail/Exception.php';
            throw new Zend_Mail_Exception('Message-ID set twice');
        }

        return $this;
    }