maildrop vs letter box

maildrop and letter box both are nouns.

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

As nouns, letter box is a hyponym of maildrop; that is, letter box is a word with a more specific, narrower meaning than maildrop:

Other hyponyms of maildrop include mailbox, postbox.

maildrop (noun)	letter box (noun)
a drop where mail can be deposited	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