box vs mailbox

box and mailbox both are nouns.

Word	Noun	Adjective	Verb	Adverb
box	Yes	No	Yes	No
mailbox	Yes	No	No	No

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

box (noun)	mailbox (noun)
a blow with the hand (usually on the ear)	a private box for delivery of mail
a (usually rectangular) container; may have a lid	public box for deposit of mail
separate partitioned area in a public place for a few people
private area in a theater or grandstand where a small group can watch the performance
the driver's seat on a coach
any one of several designated areas on a ball field where the batter or catcher or coaches are positioned
evergreen shrubs or small trees
the quantity contained in a box
a rectangular drawing
a predicament from which a skillful or graceful escape is impossible