From early August 2012 to the end of November 2013 I was stationed in Pyongyang (DPRK) for the World Food Program of the United Nations. Because of the limited international internet access allowed to the WFP national staff in the DPRK (mainly seconded by the Ministry of Foreign Affairs). There is also a WFP DPRK Support Office in Beijing, and all the more serious incoming and outgoing mail is routed through this office by diplomatic bag (transported via DHL or travelling international staff members).
The organization also had a P.O. Box in Pyongyang at the International Post Office based at the International Communications Center. Incoming mail at our P.O. Box included especially many catalogues and special sales flyers from Peter Justesen, a Danish company selling duty-free food and non-food stuff to diplomats, and later on, also the mail I was sending to myself! All incoming mail receives a Pyongyang cancellation.