[exim] deny hosts problem with IPs that don't resolve to hos…

Hi,

I have a gateway mail server which scans and filters messages
and then passes them onto an exim server... I've applied the
following ACL on the exim server to stop it accepting any mail
that isn't from the gateway or someone who has POP-before-SMTP'ed):

deny hosts = !my-gatewayIP : !net-lsearch;/var/log/pop-ips.log
message = Please deliver the message via MX - not directly to this server!

It works sometimes - it blocked 82.253.73.79
(lns-bzn-38-82-253-73-79.adsl.proxad.net)
from sending some viruses directly to the hostname but it lets
other messages through (eg from 203.154.117.2):

>>> host in "!my-gatewayIP : !net-lsearch;/var/log/pop-ips.log"?
no (failed to find host name for 203.154.117.2)
>>> deny: condition test failed
>>> processing "accept"

The error is curious because I can find a hostname for that IP ....
but regardless how do I stop it allowing by default?

Thanks,
Adrian.