public function create_default_folders()
    {
        $rcube = rcube::get_instance();

        // create default folders if they do not exist
        foreach (self::$folder_types as $type) {
            if ($folder = $rcube->config->get($type . '_mbox')) {
                if (!$this->folder_exists($folder)) {
                    $this->create_folder($folder, true, $type);
                }
                else if (!$this->folder_exists($folder, true)) {
                    $this->subscribe($folder);
                }
            }
        }
    }