first-class mail vs first class

first-class mail and first class both are nouns.

Word	Noun	Adjective	Verb	Adverb
first-class mail	Yes	No	No	No
first class	Yes	No	No	Yes

As nouns, first-class mail and first class are synonyms defined as:

first-class mail and first class: mail that includes letters and postcards and packages sealed against inspection

Other synonyms of first-class mail include 1st-class mail, 1st class.

first-class mail (noun)	first class (noun)
mail that includes letters and postcards and packages sealed against inspection	the most expensive accommodations on a ship or train or plane
	the highest rank in a classification
	mail that includes letters and postcards and packages sealed against inspection

first-class mail (adverb)	first class (adverb)
	by first class conveyance; with first class accommodations