From Longman Dictionary of Contemporary Englishldoce_268_apostboxpost‧box /ˈpəʊstbɒks $ ˈpoʊstbɑːks/ noun [countable] British English 
TCMa box in a public place, into which you put letters that you want to send SYN letterbox British English, mailbox American English
TCMa box in a public place, into which you put letters that you want to send SYN letterbox British English, mailbox American EnglishExamples from the Corpus
postbox• It hit a postbox before ploughing into the wall.• To the left of the doorway is the first postbox in Milan, dating back to the Napoleonic era.• He raised it, threw it over the small postbox gate.• As mummy stepped out of the car park, tugging Anna's arm even though she was hurrying, she turned away from the postbox.• Madeleine gave her a quick kiss, then tossed her the postbox keys.• Secure on to the front of the postbox. 