Feldpost is the German

Germany

Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

 military mail

Military mail

A primary feature of military mail systems is that normally they are subsidized to ensure that military mail posted between duty stations abroad and the home country does not cost the sender any more than normal domestic mail traffic...

 service. Its history dates back to the 18th century in the Kingdom of Prussia

Prussia

Prussia was a German kingdom and historic state originating out of the Duchy of Prussia and the Margraviate of Brandenburg. For centuries, the House of Hohenzollern ruled Prussia, successfully expanding its size by way of an unusually well-organized and effective army. Prussia shaped the history...

 during the Seven Years' War

Seven Years' War

The Seven Years' War was a global military war between 1756 and 1763, involving most of the great powers of the time and affecting Europe, North America, Central America, the West African coast, India, and the Philippines...

 and War of the Bavarian Succession and has existed ever since in different forms and shapes.
During 1937-39, the German Wehrmacht

Wehrmacht

The Wehrmacht – from , to defend and , the might/power) were the unified armed forces of Nazi Germany from 1935 to 1945. It consisted of the Heer , the Kriegsmarine and the Luftwaffe .-Origin and use of the term:...

 had a military mailing service that provided free postal services within Germany. On September 3, 1939, the first changes to the service occurred.