public function build_folder_tree(&$arrFolders, $folder, $delm = '/', $path = '')
    {
        // Handle namespace prefix
        $prefix = '';
        if (!$path) {
            $n_folder = $folder;
            $folder   = $this->storage->mod_folder($folder);

            if ($n_folder != $folder) {
                $prefix = substr($n_folder, 0, -strlen($folder));
            }
        }

        $pos = strpos($folder, $delm);

        if ($pos !== false) {
            $subFolders    = substr($folder, $pos+1);
            $currentFolder = substr($folder, 0, $pos);

            // sometimes folder has a delimiter as the last character
            if (!strlen($subFolders)) {
                $virtual = false;
            }
            else if (!isset($arrFolders[$currentFolder])) {
                $virtual = true;
            }
            else {
                $virtual = $arrFolders[$currentFolder]['virtual'];
            }
        }
        else {
            $subFolders    = false;
            $currentFolder = $folder;
            $virtual       = false;
        }

        $path .= $prefix . $currentFolder;

        if (!isset($arrFolders[$currentFolder])) {
            $arrFolders[$currentFolder] = array(
                'id'      => $path,
                'name'    => rcube_charset::convert($currentFolder, 'UTF7-IMAP'),
                'virtual' => $virtual,
                'folders' => array()
            );
        }
        else {
            $arrFolders[$currentFolder]['virtual'] = $virtual;
        }

        if (strlen($subFolders)) {
            $this->build_folder_tree($arrFolders[$currentFolder]['folders'], $subFolders, $delm, $path.$delm);
        }
    }