[Exim] host_reject_recipients logging problem

I have a network that I'm blocking but they don't have any PTR RRs. Thus
when I try ./exim -bh ip_address

(where this is member of said netblock), I see
>>> host in host_reject_recipients? yes (204.157.168.0/24 in /disk2/exim/tables/
blocked-networks.real)
LOG: recipients from [204.157.168.1] refused (failed to find host name from IP a
ddress)
...
550-contact postmaster@??? for details
550 mail from 204.157.168.1 rejected: administrative prohibition (failed to find
host name from IP address)

Now while this is true, I'm not blocking sites that fail to have PTR RRs
and even if they fix their DNS, they will still be blocked. Why does exim
(3.30) do this? Is it because I've already done a host lookup (which has
failed) and this somehow overrides the explicit block?

Thanks

--
Alan Thew
FAX: +44 151 794 4442