A lot of mail can be found to passengers (or workers) on board of a ship. Quite a number of them are returned to sender or redirected, mainly because the person is not on board or because the ship has left.
In this case the postcard was sent from Amsterdam on 16.7.1897 to a person on board of the s.s. Mars. Returned to sender it arrived again in Amsterdam on 3.8.1897. In German it states that the steamer has been sent to Amsterdam. The Mars was a steamer of the Koninklijke Nederlandsche Stoomboot Maatschappij, Amsterdam / Royal Netherlands Steamship Co. and was built in in 1880 and sold to Italy in 1922.