A long time back, we found that none of our products and services are applicable to the Chinese and Korean market. However my mail servers continued to get a lot of spam from these sources – which was weird!
At some point I came across Okean (www.okean.com) and started using their list to prevent access from such sources. I find it works great; and here’s the NETSH command used to perform the block on a windows box:
netsh advfirewall firewall add rule name="IP Block" dir=in interface=any action=block remoteip=x.x.x.x/32