[exim] How to be resilient to mysql server unreachable?

Hi list, im planning to implement a vacation message using a mysql
server, and i would like that when the mysql server were down, the exim
continues delivering messages and ignoring the vacation (failing the
lookup). The mysql server only has the vacation info, the rest are done
by local files and unix users.
I didnt find a clear answer in the documentation (section 9.5) about
what happens when a lookup can't reach the sql server (temporary errors?
its use the retry configuration? will drop messages or fail the look up?)

This is what I added to the configure file:

vacation_router:
        driver = accept
        check_local_user

        condition = ${lookup mysql \
{SELECT mailbox FROM vacation\
WHERE mailbox='${quote_mysql:$local_part}'} {true }{false}}

        transport = vacation_transport
        unseen

vacation_transport:

        driver = autoreply

        from = ${lookup mysql \
{SELECT remitente FROM vacation\
 WHERE mailbox='${quote_mysql:$local_part}'}}

        subject = "Respuesta automatica"

        text = "${lookup mysql\
{SELECT texto FROM vacation\
WHERE mailbox='${quote_mysql:$local_part}'}}"

        to = $sender_address
        once = $home/vacation.db
        once_repeat = 7d

Thanks. Juan.