[exim] Exim fails if Database is not available

Hello,

i am runnig Exim 4.34 with a mysql database. In this database are all
information for Exim.
If the database is to available and somebody send a email, the sender get a
message like that.

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its recipients.
This is a permanent error. The following address(es) failed:

  exim@???
    Expansion of "${lookup mysql{SELECT vacationSubject FROM user WHERE
email="${local_part}@${domain}"}{$value}fail}" failed in vacation_user
transport:
    "lookup" failed and "fail" requested

Is it possible that the exim don´t accept connections if the databaseconnection
failed, so that the user don´t notice that the exim have an error?

thx

Kai