[exim] Spam filtering of blacklisted name serers

I'm having luck looking up the name servers of the sending host to see
if it's blacklisted

    set acl_c_ns = ${lookup 
dnsdb{>;ns=$acl_c_sender_host_domain}{$value}fail}
    dnslists=hostkarma.junkemailfilter.com=127.0.0.2/<;$acl_c_ns

In this case $acl_c_sender_host_domain is the registry barrier pary of
the host name (ie example.com)

Anyhow - my point - it's catching a lot of spam especially from new
domains just registered. Something worth exploring.