mail service vs airmail

mail service and airmail both are nouns.

Word	Noun	Adjective	Verb	Adverb
mail service	Yes	No	No	No
airmail	Yes	No	Yes	No

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

mail service: the system whereby messages are transmitted via the post office
airmail: a system of conveying mail by aircraft

Other hyponyms of mail service include airpost, snail mail, RFD, rural free delivery.

mail service (noun)	airmail (noun)
the system whereby messages are transmitted via the post office	a system of conveying mail by aircraft
	letters and packages that are transported by aircraft

mail service (verb)	airmail (verb)
	send or transport by airmail