I don't know how to find the information to do so, but if countries are given contiguous blocks of IP addresses, one could then block by creating a rule (or a set of rules) to look for, as an example, "If Received contains \[85. then do something." (Look at a message with long headers and you'll see to what I'm alluding.) When building the rule, first choose "edit header list" in the first top pulldown menu and add "Received" to the list, then it will show up with the other choices of To, From, CC, and Subject. I'm thinking that might help to get around possibly forged addresses that don't end in ".cn" or whatever, but should.
Could this be something that your email provider could implement on the server side?