dead mail vs dead letter

dead mail and dead letter both are nouns.

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

As nouns, dead mail and dead letter are synonyms defined as:

dead mail and dead letter: mail that can neither be delivered nor returned

dead mail (noun)	dead letter (noun)
mail that can neither be delivered nor returned	mail that can neither be delivered nor returned
	the state of something that has outlived its relevance