Update (9.29.2014): a much better solution is to implement a spam-blocking service like zen on spamhaus.org.
This solution has been incredible for cutting down on the number of spam emails I receive. I would say based on the hits I get on my mail servers from spam bots, about 95% of the spam that attempts to hit my mail server comes from over-seas. I mean who am I going to contact in Belgium, South Korea, and Peru anyway? And if someone has to get a hold of me from over-seas, they can just go to https://www.westerfunk.net/contact/email/ and send me an email. In addition, most of the people I know who would contact me over-seas could just email me using an American-based email provider (e.g. Yahoo, Gmail, AOL, Hotmail, etc). So this solution has been awesome for my personal email setup. I do understand this won’t work for larger organizations, but for those of you who run your own mail server and don’t have foreign contacts, give it a whirl. Here are the foreign subnets I looked up and banned at the mail server level (based off of this info @ http://www.iana.org/assignments/ipv4-address-space):
188.8.131.52 – 184.108.40.206 (1 Subnet – Africa)
220.127.116.11 – 18.104.22.168 (1 Subnet – Japan)
22.214.171.124 – 126.96.36.199 (5 Subnets – Asia and Europe)
188.8.131.52 – 184.108.40.206 (15 Subnets – Europe)
220.127.116.11 – 18.104.22.168 (6 Subnets – Asia)
22.214.171.124 – 126.96.36.199 (2 Subnets – Latin America)
188.8.131.52 – 184.108.40.206 (3 Subnets – Asia and Europe)
220.127.116.11 – 18.104.22.168 (4 Subnets – Asia and Latin America)
22.214.171.124 – 126.96.36.199 (4 Subnets – Asia and Europe)
188.8.131.52 – 184.108.40.206 (6 Subnets – Asia and Europe)
This pretty much takes care of most foreign IP blocks. But you have to remember that these are country-registered IP blocks and may not cover every IP registered in a foreign country. Regardless, the only spam I have received up to this point has been from American IP’s that are not blacklisted on the spam database filters (such as www.spamhaus.org). But even that has become rare now as it seems more people are reporting spam hits and from what IP it originated from.