mail boat vs boat

mail boat and boat both are nouns.

Word	Noun	Adjective	Verb	Adverb
mail boat	Yes	No	No	No
boat	Yes	No	Yes	No

As nouns, boat is a hypernym of mail boat; that is, boat is a word with a broader meaning than mail boat:

mail boat (noun)	boat (noun)
a boat for carrying mail	a small vessel for travel on water
	a dish (often boat-shaped) for serving gravy or sauce

mail boat (verb)	boat (verb)
	ride in a boat on water