In the latter part of the 19th and early part of the 20th century, a number of countries maintained post offices in foreign countries, arranged by treaty. Most such offices were operated by European powers in the Middle and Far East. They were partly motivated by the desire to provide reliable postal service for merchants and other foreign nationals in major cities, and partly by suspicion of the local postal service. The currency in use could be either the local currency, or that of the home country.
Upper stamps were issued for Polish post offices in Turkey. Mail was accepted at the Polish consulate in Constantinople, 1919-1923.