Lobby of the Washington City Post Office Building, by Penland, Dane A, 1992, Smithsonian Archives - History Div, The Historic Lobby of the Old Washington City Post Office Building leading to the new home for the National Postal Museum. A portion of the lower level of the palatial beaux arts quarters of the building became the new home for the National Postal Museum. Opened in 1993, the museum was established in 1990 under an agreement between the Smithsonian Institution and the United States Postal Service..