There was a post office on the Russian space station Mir. Visiting cosmonauts would use unique postal "markers" to stamp envelopes and other items as having flown aboard the Mir space station

Assembled in orbit from 1986 to 1996, Mir was the first modular space station and had a greater mass than that of any previous spacecraft, holding the record for the largest artificial satellite orbiting the Earth until its deorbit on 21 March 2001 (a record now surpassed by the International Space Station). Mir served as a microgravity research laboratory in which crews conducted experiments in biology, human biology, physics, astronomy, meteorology and spacecraft systems in order to develop technologies required for the permanent occupation of space.
The station was the first consistently inhabited long-term research station in space and was operated by a series of long-duration crews. The Mir programme held the record for the longest uninterrupted human presence in space, at 3,644 days, until 23 October 2010 (when it was surpassed by the ISS), and it currently holds the record for the longest single human spaceflight, of Valeri Polyakov, at 437 days 18 hours. Mir was occupied for a total of twelve and a half years of its fifteen-year lifespan, having the capacity to support a resident crew of three, and larger crews for short-term visits.
Following the success of the Salyut programme, Mir represented the next stage in the Soviet Union’s space station programme. The first module of the station, known as the core module or base block, was launched in 1986, and was followed by six further modules, all launched by Proton rockets (with the exception of the docking module). When complete, the station consisted of seven pressurised modules and several unpressurised components. Power was provided by several photovoltaic arrays mounted directly on the modules. The station was maintained at an orbit between 296 km (184 mi) and 421 km (262 mi) altitude and traveled at an average speed of 27,700 km/h (17,200 mph), completing 15.7 orbits per day.
The station was launched as part of the Soviet Union’s manned spaceflight programme effort to maintain a long-term research outpost in space, and, following the collapse of the USSR, was operated by the new Russian Federal Space Agency (RKA). As a result, the vast majority of the station’s crew were Soviet or Russian; however, through international collaborations, including the Intercosmos, Euromir and Shuttle-Mir programmes, the station was made accessible to astronauts from North America, several European nations and Japan. The cost of the Mir programme was estimated by former RKA General Director Yuri Koptev in 2001 as $4.2 billion over its lifetime (including development, assembly and orbital operation). The station was serviced by Soyuz spacecraft, Progress spacecraft and U.S. space shuttles, and was visited by astronauts and cosmonauts from 12 different nations.