mail vs express

mail and express both are nouns.

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

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

mail: the bags of letters and packages that are transported by the postal service
express: mail that is distributed by a rapid and efficient system

As verbs, express is a hyponym of mail; that is, express is a word with a more specific, narrower meaning than mail:

Other hyponyms of mail include airmail, register, express-mail.

mail (noun)	express (noun)
(Middle Ages) flexible armor made of interlinked metal rings	rapid transport of goods
a conveyance that transports the letters and packages that are conveyed by the postal system	public transport consisting of a fast train or bus that makes only a few scheduled stops
the system whereby messages are transmitted via the post office	mail that is distributed by a rapid and efficient system
the bags of letters and packages that are transported by the postal service
any particular collection of letters or packages that is delivered

mail (adjective)	express (adjective)
	not tacit or implied
	without unnecessary stops

mail (verb)	express (verb)
cause to be directed or transmitted to another place	articulate; either verbally or with a cry, shout, or noise
send via the postal service	give expression to
	serve as a means for expressing something
	indicate through a symbol, formula, etc.
	send by rapid transport or special messenger service
	obtain from a substance, as by mechanical action
	manifest the effects of (a gene or genetic trait)

mail (adverb)	express (adverb)
	by express