There is a country in Postcrossing that is not a real country!

Postcrossing follows the ISO standard 3166 for the 2-letter country codes used in the Postcard IDs. This ISO standard covers not only sovereign countries, but also dependent territories and regions of special interest.

Taiwan, Macao, Hong Kong, Åland Islands, Wallis and Futuna, Cayman Islands, Christmas Island etc. all fall into this category.