[exim] temporarily rejected RCPT caused by MySQL failed conn…

We currently maintain a local blacklist in a mysql database. However
when the database went down we stopped receiving mail. Once the
database was restarted we began receiving mail again. All log messages
were of this type:

F=<xxxxx@fake_domain.com> temporarily rejected RCPT <xxxxx@???>:
MYSQL connection failed: Can't connect to MySQL server on '127.0.0.1' (111)

Is there a way for exim to continue to accept messages if it cannot
connect to the database? Something similar to an RBL lookup that has
failed or timed out?

DNS list lookup defer (probably timeout) for xx.xx.xx.xx.
brazil.blackholes.us: assumed not in list

MYSQL query:
condition = ${lookup mysql{select ip_address from blacklist where
ip_address='$sender_host_address'}{yes}{no}}