noun
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
郵便屋さんがドアポストに手紙を入れました。
The mail carrier put a letter into the door-mounted mailbox.
このマンションの各部屋にはドアポストが付いています。
Each room in this apartment building has a letterbox on the door.