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

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

record (a widescreen film) on to video in letterbox format

the film has been released in a digitally remastered and letterboxed edition