letter box vs PO Box

letter box and PO Box both are nouns.

Word	Noun	Adjective	Verb	Adverb
letter box	Yes	No	No	No
PO Box	Yes	No	No	No

As nouns, letter box and PO Box are synonyms defined as:

letter box and PO Box: a numbered compartment in a post office where mail is put to be called for

Other synonyms of letter box include call box, POB, Post-Office box.

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