我把信封塞进了门上的信箱口。

I pushed the envelope through the ___letterbox in the door.

这部电影以宽屏格式播放，上下有黑边。

The movie is shown in ___letterbox format with black bars on top and bottom.

这部宽屏电影需要经过信箱化处理才能在普通电视上播放。

They had to ___letterbox the widescreen movie to make it fit regular TVs.