[Exim] This might be a useful spam catching trick

I don't know how useful this will be in catching spam but it might work
well with anything that has a basian filter, like Spam Assassin.

What I'm doing is a lookup on the sending host's IP address to see who
the name server authority is for the IP itself and putting it into a
header. The idea is that name servers for spammers will be different
than name servers for non-spammers and the the filter will see this
information and start to learn from it.

I'm guessing this will be a useful tool.

headers add "X-Sender-Nameserver: ${lookup
dnsdb{ns=${sg{$sender_host_address}\
{\\N([0-9]{1,3})\.([0-9]{1,3})\.([0-9]{1,3}\)\.([0-9]{1,3}\)\\N}\
{\\N$3.$2.$1.in-addr.arpa\\N}}}}"