Beginning in late 2001, we started receiving vast amounts of junk e-mail (better known as spam) from Korean networks. Some of it was in Korean, a language that nobody here understands, some was in English. Despite repeated attempts to contact system managers to alert them to the problem, we never received even a single acknowledgement to any of our reports.
So, with regret, we have blocked mail from most South Korean networks.
The blocking mechanism is a DNSBL (DNS blocking list) called korea.services.net. It lists the numeric addresses of Korean networks, in a form that mail systems can check efficiently.