Transnistria (officially the Pridnestrovian Moldavian Republic, and also called Pridnestrovie) is de facto independent, but an unrecognized country in Europe. It is squeezed between Moldova, which they fought against for independence, and Ukraine.
Transnistria does have it’s own postal system, and issues own stamps. But it only works inside the country. To send international mail Moldovan postal service is used (both domestic and Moldovan stamps are available in Transnistria’s postal offices). Same goes for incoming international mail.
So, if you want to send something to Transnistria, you should specify Moldova as a destination country. Otherwise it will not be delivered.