dead mail vs letter

dead mail and letter both are nouns.

Word	Noun	Adjective	Verb	Adverb
dead mail	Yes	No	No	No
letter	Yes	No	Yes	No

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

Other hypernyms of dead mail include missive.

dead mail (noun)	letter (noun)
mail that can neither be delivered nor returned	a written message addressed to a person or organization
	the conventional characters of the alphabet used to represent speech
	an award earned by participation in a school sport
	a strictly literal interpretation (as distinct from the intention)
	owner who lets another person use something (housing usually) for hire

dead mail (verb)	letter (verb)
	mark letters on or mark with letters
	set down or print with letters
	win an athletic letter