protected function _loadMailbox($mailbox, $uidvalid = null)
    {
        if (!isset($this->_mbox[$mailbox]) &&
            ($ob = $this->_hash->get($this->_getCid($mailbox)))) {
            try {
                $this->_mbox[$mailbox] = $this->_pack->unpack($ob);
            } catch (Horde_Pack_Exception $e) {}
        }

        if (isset($this->_mbox[$mailbox])) {
            if (is_null($uidvalid) ||
                ($uidvalid == $this->_mbox[$mailbox]['d']['uidvalid'])) {
                return;
            }
            $this->deleteMailbox($mailbox);
        }

        $this->_mbox[$mailbox] = array(
            // Metadata storage
            // By default includes UIDVALIDITY of mailbox.
            'd' => array('uidvalid' => $uidvalid),
            // List of UIDs
            'u' => new Horde_Imap_Client_Ids()
        );
    }