An uncommon, somewhat literary term for a box or container for letters. In modern Japanese, 郵便受け or ポスト are far more common.
A letterbox was attached to the gate of the old mansion.
book chest; book box
Refers to a chest or box for storing books, often used historically. Not a common word in everyday speech.
Several old book chests were piled up in the corner of the library.
post office box
Compound of 書 (writing, book) and 函 (box, container). The exact historical derivation is uncertain, but the word follows a classical Chinese pattern for naming containers.