door-mounted mailbox; letterbox on the front door
A mailbox attached to or built into the front door of a house or apartment, common in Japan. Note that this is wasei-eigo; the English 'door post' does not carry this meaning.
wasei-eigo (Japanese-coined English), from door post
The mail carrier put a letter into the door-mounted mailbox.
Each room in this apartment building has a letterbox on the door.
Variant with interpunct; less common but seen in some contexts.
Wasei-eigo (Japanese-made English) from 'door' and 'post', referring to a mailbox on the door. Not used in English with this meaning.