public function folder_namespace($folder)
    {
        if ($folder == 'INBOX') {
            return 'personal';
        }

        foreach ($this->namespace as $type => $namespace) {
            if (is_array($namespace)) {
                foreach ($namespace as $ns) {
                    if ($len = strlen($ns[0])) {
                        if (($len > 1 && $folder == substr($ns[0], 0, -1))
                            || strpos($folder, $ns[0]) === 0
                        ) {
                            return $type;
                        }
                    }
                }
            }
        }

        return 'personal';
    }