0 a rectangular hole in the door or in a wall near the entrance of a house or other building, through which letters, etc. are put -- （房屋或其他建筑物入口处的门或墙上的）信箱
Gillie witnesses the incident through the mail slot in the apartment door.
Mail slots are limited to receiving incoming mail, as most have no provision for securing and protecting outgoing mail for pickup by the mail carrier.
Wall-mounted or attached mailboxes may also be used in place of mail slots, usually located close to the front door of the residence.
These were sometimes referred to as mail slot windows.
The plan contained a single vertical circulation core with four elevators, a mail slot, and staircase.
This new design also incorporates a parcel locker and an outgoing mail slot for resident convenience.
