A mail chute is a largely defunct letter collection device used in early multi-storey office buildings, hotels, apartment buildings and other high rise structures. Letters were dropped from the upper storeys and collected (usually at the ground level) at a central depository by the postal service. This innovation was before the time of the modern "mail room" normally associated nowadays with high rise buildings. It was for the convenience of the users of the building so they would not have to take their mail to an outside mail box or to the post office.
Sheffield Weekly Telegraph - Saturday 3 June 1911
Letter Chutes While waiting for the lift in almost any skyscraper building in the States, the stranger notices the words “U.S. Mail Chute.” Let him wait a little longer and there will be a flash of white pass, like lightning, and on looking closely he finds himself in front of a novel letterposting arrangement. There is one letter-box for the whole building (sometimes two), and it consists of a big receptacle on the ground floor supplied by a chute, with a glass front, that begins at the highest offices on the top floor. One by one the letters dart down, flashes of whiteness, to fall into the box below. It is certainly cute arrangement, saving much time, though demanding, strong stationery, for when a fairly heavy envelope drops a hundred feet or two from the top floor it needs to be made of wiry paper and to be well sealed if it is to reach its journey’s end in safety. In the large blocks of flats, where the multitude live, the postman places the tenants’ letters in little brass receptacle just under tho bell number. Sometimes the letters are too numerous to go in, so they are left sticking out, open to the thieving instincts of an unscrupulous passer-by. Letters, however, seem to be left alone, and although thieves abound and robberies are frequent, one does not hear of postal pilferings.