A very large percentage of the spam I receive comes from mailservers in China and Korea. As I don't know anyone there, and don't expect any email from there, I've implemented a Great Wall of Spam to prevent Chinese and Korean mailservers from sending data to my mailserver.

Rather than use the full list that Steve uses I've just blocked all the IPs in China and Korea with IPTables using these rules.

We'll see how that goes in reducing the incoming spam volume from the current 100/day or so to my main email address, and then think about adding Brazil. I'm also worried about BotNets of compromised machines in the US and elsewhere, but I'm not sure whether that's fixable at this level.