mail vs express-mail

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

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

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

Other hyponyms of mail include express, airmail, register.

mail (noun)	express-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

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