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1British a box attached to an outside wall, or a slot in the door of a building, into which mail is delivered: she wrote a hasty reply and posted it through his letter box
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2 (letterbox) [mass noun, usually as modifier] a format for presenting widescreen films on a standard television screen, in which the image is displayed in approximately its original proportions across the middle of the screen, leaving horizontal black bands above and below: this uncut version is presented in letterbox format