[exim] checking for missing PTR custom condition

Hi list,

I wonder if anyone can point me in the right direction for testing if
the sending host has a missing PTR record, but not doing the default
host_lookup which will then do a double check that there is a matching A
record. My reason being, I want to be able to give different
Spamassassin scores to each one, no matching A record is bad but not as
bad as no PTR at all.

I didn't find any examples of anyone else doing exactly this but I
cobled together this condition, which I've done wrong and doesn't work:

# warn
# condition = ${if !lookup dnsdb{ptr=$sender_address_domain}}
# add_header = X-Host-Lookup-Failed-Test: Reverse DNS lookup failed for
$
sender_address_domain

It says: unknown condition "lookup"

Any tips on how to achieve this?

thanks in advance, Andy.