letter vs dead mail

letter and dead mail both are nouns.

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

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

letter (noun)	dead mail (noun)
a written message addressed to a person or organization	mail that can neither be delivered nor returned
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

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