Introduction to the official stamps of Poland
Official mail is all mail that pertains exclusively to the official correspondence of a government. Official stamps are also known as Service stamps. They are called "Dienstmarken" (Service stamps) in German. Government business mail was sent by government officials without postage prepayment and was valid only on official mail of a government agency or their agents.
Polish official stamps can be recognised by the inscription "Przesyłka Urzędowa" or the abbreviation "U".