def getquota(mailbox)
      synchronize do
        send_command("GETQUOTA", mailbox)
        return @responses.delete("QUOTA")
      end
    end
Sends the GETQUOTA command along with specified +mailbox+.
If this mailbox exists, then an array containing a
Net::IMAP::MailboxQuota object is returned.  This
command generally is only available to server admin.