The other day, I stumbled across a handy-dandy table on the Internet somewhere that has a list of IP ranges and the country that they are reserved for. Just for fun, I wrote a script that lets me enter in an IP and then the country where it is assigned pops back to the screen. I then randomly sampled 2500 IPs that sent us a lot of spam and then mapped those results to a map of the world.
The below diagram is the result of that mapping. The worst offenders should surprise no one - the US and China. This is concentration by IP, not by total mail from these IPs.
The top five worst countries are the US, China, India (surprising), Turkey and Brazil. These 5 combined accounted for half of our spam on this one particular day.
I thought that these results were interesting.