[exim] warn ACL and deferred LDAP lookup

Hi,

I would like to use a warn ACL to set an ACL variable depending on the
result of a LDAP lookup using ${lookup ldapm{...}}. I use something
similar like this:

warn condition = ${lookup ldapm{...}}
     set acl_c0 = 1

This works just fine, except for one thing: In case the lookup fails for
some reason (e.g. the LDAP server is currently unreachable), I would
like the entire ACL to return defer and *not* continue with the next
ACL. How do I achieve that?

Regards,
Micha