The Chinese word for letter box is
信箱

xìn xiāng

Chinese Definition

Components
信
1. true; accurate; verifiable
2. truly; indeed
3. trustworthy
4. to believe; to trust in something
我才不信呢。 - I don't believe it/you.
信任 - to trust -
5. to believe in (a faith); to be religious in
我父親信佛教。 - My father is a Buddhist. (lit. My father believes in Buddhism.)
6. trust; confidence; faith
因信稱義 - to be justified by faith -
7. at will
信口.雌黃 - to make irresponsible remarks (lit. to correct someone with at will) -
8. letter (written communication)
9. message; information; news
信息 - message -
10. evidence
信物 - token, keepsake -
11. courier; messenger
12. Short for 信州
13. (mineral) Short for 信石, arsenolite
Xin
14. Alternative form of 申 (to express; to state)
15. Alternative form of 伸 (to extend)
16. Alternative form of 身 (body)
箱
1. (large) box; chest; trunk
拎住一大箱嘢 - carrying a great box of things
2. box-shaped thing
3. Alternative form of 廂 (side room)