Definition of letterboxes:
(n) :
(chiefly UK, Ireland, Commonwealth) A box into which mail is put.
(n) :
A collection point for mail intended for onward delivery, a secure box or receptacle for this purpose.
(n) :
A delivery point for mail, a box, compartment or slot for this purpose.
(adj) :
(film) Transferred to home video formats while preserving the original aspect ratio, having black bars above and below the picture area.
(v) :
(film, transitive) To transfer a widescreen motion picture to home video formats while preserving the original aspect ratio, with the placing of black bars above and below the picture area.
(n) :
A hidden container that holds a logbook and rubber stamp, found by following clues as a form of recreation.