[Exim] This might be a useful spam catching trick - Improved

# Tag message with sender's IP nameserver record.
# This can be processed with Spam Assassin
# Does IP NS lookup (W.X.Y.Z) on Z.Y.X.in-addr.arpa and Z.Y.in-addr.arpa

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