protected function _createTmpFile($folder = 'INBOX')
    {
        if ($folder == 'INBOX') {
            $tmpdir = $this->_rootdir . DIRECTORY_SEPARATOR . 'tmp' . DIRECTORY_SEPARATOR;
        } else {
            $tmpdir = $this->_rootdir . '.' . $folder . DIRECTORY_SEPARATOR . 'tmp' . DIRECTORY_SEPARATOR;
        }
        if (!file_exists($tmpdir)) {
            if (!mkdir($tmpdir)) {
                /**
                 * @see Zend_Mail_Storage_Exception
                 */
                // require_once 'Zend/Mail/Storage/Exception.php';
                throw new Zend_Mail_Storage_Exception('problems creating tmp dir');
            }
        }

        // we should retry to create a unique id if a file with the same name exists
        // to avoid a script timeout we only wait 1 second (instead of 2) and stop
        // after a defined retry count
        // if you change this variable take into account that it can take up to $max_tries seconds
        // normally we should have a valid unique name after the first try, we're just following the "standard" here
        $max_tries = 5;
        for ($i = 0; $i < $max_tries; ++$i) {
            $uniq = $this->_createUniqueId();
            if (!file_exists($tmpdir . $uniq)) {
                // here is the race condition! - as defined in the standard
                // to avoid having a long time between stat()ing the file and creating it we're opening it here
                // to mark the filename as taken
                $fh = fopen($tmpdir . $uniq, 'w');
                if (!$fh) {
                    /**
                     * @see Zend_Mail_Storage_Exception
                     */
                    // require_once 'Zend/Mail/Storage/Exception.php';
                    throw new Zend_Mail_Storage_Exception('could not open temp file');
                }
                break;
            }
            sleep(1);
        }

        if (!$fh) {
            /**
             * @see Zend_Mail_Storage_Exception
             */
            // require_once 'Zend/Mail/Storage/Exception.php';
            throw new Zend_Mail_Storage_Exception("tried $max_tries unique ids for a temp file, but all were taken"
                                                . ' - giving up');
        }

        return array('dirname' => $this->_rootdir . '.' . $folder, 'uniq' => $uniq, 'filename' => $tmpdir . $uniq,
                     'handle' => $fh);
    }