- It is a kind of Ship Equipment.
Mail Vaults come in vastly different sizes from small hand vaults to large holds capable of taking cubic feet of mail carts. For some off reason, many mail systems still use pre-metric Terran metrics (...measurement systems). 
The sheer size of the Third Imperium means that ordinary communications must depend on ships travelling along the main trade routes, making Regina (Regina / Spinward Marches 1910) nearly four years out from the Imperial Core Sector.
The express boat (...abbreviated X-boat) system, established originally in 624 and expanded to cover the entire Imperium by 718, cuts this communication time by nearly 75 percent. Selected locations along major trade routes are established as sites for express stations, which are orbital facilities which service and refuel the X-boats on their communications runs. Often an express boat tender serves as the station.