a. a slot, usually covered with a hinged flap, through which letters, etc are delivered to a building
b. Also called (in Britain and certain other countries): letter box a private box into which letters, etc, are delivered
2.
chiefly (US), (Canadian) Also called (in Britain and certain other countries): postbox a public box into which letters, etc, are put for collection and delivery
3.
(on a computer) the directory in which e-mail messages are stored; also used of the icon that can be clicked to provide access to e-mails