[Exim] exceptions to host_lookup

I want to exempt certain networks or hosts from lookups, mainly because
they have a problem with their DNS and the host lookup does not yield a
correct hostname.

This is what I put in my exim.conf (Exim ver 3.02):

host_lookup = "!/var/mail/block/host_lookup-except : \
     !/var/mail/block/net_lookup-except : \
     0.0.0.0/0"

Does this have any chance of working? It appears to work when I test it,
to the extent that 'exim -bh <ip_add_ress>' shows that the exception was
located, but it also appears that the lookup if performed anyway.

thanks

--
Andrew