protected function _mailboxOb($mailbox = null)
    {
        $name = is_null($mailbox)
            ? strval($this->_selected)
            : strval($mailbox);

        if (!isset($this->_temp['mailbox_ob'][$name])) {
            $this->_temp['mailbox_ob'][$name] = new Horde_Imap_Client_Base_Mailbox();
        }

        return $this->_temp['mailbox_ob'][$name];
    }