PO Box vs letter box

PO Box and letter box both are nouns.

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

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

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

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

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