postbox vs letter box

postbox and letter box both are nouns.

Word	Noun	Adjective	Verb	Adverb
postbox	Yes	No	No	No
letter box	Yes	No	No	No

As nouns, postbox and letter box are synonyms defined as:

Other synonyms of postbox include mailbox.

postbox (noun)	letter box (noun)
public box for deposit of mail	a private box for delivery of mail
	public box for deposit of mail
	a numbered compartment in a post office where mail is put to be called for