I recently discovered a wonderful DNSBL service reporting you where public Internet IPs are from.
The service is countries.nerd.dk: http://countries.nerd.dk/more.html
You can for example block any mail at your mailserver coming from china or russia, simply integrating this DNSBL with your MTA.
You can even get the country of your IP with dig!
Warning: You need to swap IP octets. If for example the IP to check is 126.96.36.199, you have to call 188.8.131.52.zz.countries.nerd.dk.
root@nbvirtdns1:/# dig TXT 184.108.40.206.zz.countries.nerd.dk
220.127.116.11.zz.countries.nerd.dk. 1047 IN TXT “it”