Sophos shares recent detailed study on SPAM

http://nakedsecurity.sophos.com/2013/10/17/dirty-dozen-spam-sending-nations-find-where-you-finished-in-our-q3-spampionship-chart/

http://securitywatch.pcmag.com/spam/317274-us-reigning-spampions

QUOTE:  Prepare yourself for the results of the latest Spampionship. SophosLabs conducted a study calculating the “Dirty Dozen” spam sending countries.  United States, Belarus, and India take the leading three spots on the “Dirty Dozen” list for countries that send the most amount of spam; consistent with what SophosLabs has seen this past year. Countries on this list aren’t necessarily spammers, but they are spam senders. Spam senders and spammers are two different things.  Spammers usually don’t send their own spam in bulk because it makes them easy to identify. Out of the ten million unwanted emails they attempt to send, one million could get sent before either recipients or data centers start resisting. At this point, spammers are blocked from using the servers and sending the remaining emails.