[exim] variable for acl messages?

Hello,

I have an ACL which uses

verify = reverse_host_lookup

While exim writes a detailed explanation to the rejectlog like this:

host lookup failed (200.50.112.61 does not match any IP address for
50-112-61.static.tie.cl)

exim writes only this very unspecific message to the SMTP output:

451 Temporary local problem - please try later

I would like to show the detailed message from exim to the SMTP session, but

message = $acl_verify_message

doesn't do it. Which variable contains the result of the
reverse_host_lookup check?

Regards
Marten