protected function _cacheContent()
    {
        // caching content if we can't fetch parts
        if ($this->_content === null && $this->_mail) {
            $this->_content = $this->_mail->getRawContent($this->_messageNum);
        }

        if (!$this->isMultipart()) {
            return;
        }

        // split content in parts
        $boundary = $this->getHeaderField('content-type', 'boundary');
        if (!$boundary) {
            /**
             * @see Zend_Mail_Exception
             */
            // require_once 'Zend/Mail/Exception.php';
            throw new Zend_Mail_Exception('no boundary found in content type to split message');
        }
        $parts = Zend_Mime_Decode::splitMessageStruct($this->_content, $boundary);
        if ($parts === null) {
            return;
        }
        $counter = 1;
        foreach ($parts as $part) {
            $this->_parts[$counter++] = new self(array('headers' => $part['header'], 'content' => $part['body']));
        }
    }