express-mail vs mail

express-mail is not a noun while mail is a noun.

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

As verbs, mail is a hypernym of express-mail; that is, mail is a word with a broader meaning than express-mail:

Other hypernyms of express-mail include post, send.

express-mail (noun)	mail (noun)
	(Middle Ages) flexible armor made of interlinked metal rings
	a conveyance that transports the letters and packages that are conveyed by the postal system
	the system whereby messages are transmitted via the post office
	the bags of letters and packages that are transported by the postal service
	any particular collection of letters or packages that is delivered

express-mail (verb)	mail (verb)
send by express mail or courier	cause to be directed or transmitted to another place
	send via the postal service