Universal Postal Union

The Universal Postal Union (UPU) is an international organization located in Bern, Switzerland that sets standards and policies for international postal services. It was established in 1874 and is now an agency of the United Nations.

Without membership, non-members have been and are still barred from using the international mails. The workaround for de facto countries of today like Transnistria is for mail to first go through a cooperative neighboring country, with and without additional postage of that country in different cases. Still there have been situations where technically illegal mail was passed through normal channels as in the case of Katanga and South Kasai.

Stamps
The UPU has franking power granted by the Swiss post office and uses its own official stamps.  File:Switz UPU pegasus.jpg|1960 20c Pegasus, hinting that your package is now somewhere in outer space. File:Switz UPU offic.jpg|1989 140c value commemorating express mail. File:Switz UPU Q.jpg|2003 90c service quality improvement. File:Switz UPU off 2012.jpg|2012 190c 25th Congress 
 * Its first issue in 1957-60 featured the emblematic UPU statue in Bern and Pegasus.
 * Beginning in 1976, it has issued definitives and commemoratives. The UPU's stamp program has been solidly conservative (23 stamps from 1957 to 2012 with none added to date) while its members continue to go hog wild, printing stamps on every topic under under the sun.