From the late 1800s a lot of Christian missionary activity took place in Korea. Given that these people usually were very well educated they inevitably wrote home. Many of these letters including the envelopes in which they were sent were saved by the recipients. In recent years with the passing of many of the post-World War II missionaries a lot of these items have come on the philatelic market.
Here is one simple example. The card was shown earlier on because of its possible paquebot cancellation links, but the address in the top left corner caught my eye as well.