mail chute
A duct for dropping letters from the upper floors of a building to a mailbox below.
A mail chute is a vertical chute built through at least one floorplate of a building that enables people on the upper floors to drop letters and postcards into a mailbox or collection bin on a floor below (usually the ground floor). They are generally built as a convenience to tenants, allowing them to send mail without making a special trip down an elevator or flight of stairs.
The chute passes vertically through at least one floor plate and carries deposited letters or postcards to a lower collection point. Collection on the ground floor is usual, but another lower floor is included.
The fixed passage distinguishes a mail chute from a mailbox alone. Its function is to let people deposit outgoing mail from an upper floor without taking it down to the collection point themselves.