[Exim] Basic ACL error

Hi,

I'm trying to set up an ACL that will reject HELO/EHLOs from servers
whose IP addresses have no PTR record. The following seems to work:

deny condition = ${lookup dnsdb{ptr=$sender_host_address}{no}{yes}}
     message   = no hostname found for $sender_host_address

However, if I want to exclude certain domains by doing this:

deny domains   = !exim.org
     condition = ${lookup dnsdb{ptr=$sender_host_address}{no}{yes}}
     message   = no hostname found for $sender_host_address

I get a "cannot test domains condition in EHLO or HELO ACL" error
logged. Debug mode produces the following:

check domains = !exim.org
deny: condition test error

I've obviously done something silly but cannot see what - any ideas?

Thanks in advance,

--
Mike Zanker
Northampton, UK
PGP Public Key: pgp@???