[Exim] host_lookup_nets_except?

While I like setting:

host_lookup_nets = 0.0.0.0/0

occasionally I would like to allow a particular IP to connect -- even if it
doesn't resolve to a host name.

Is there a configuration setting that could override the above setting?
(Something like "host_lookup_nets_except"?)

I'm using Exim 2.05. Thanks for any pointers!

Tod
abl.com