mail service vs airpost

mail service and airpost both are nouns.

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

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

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

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

mail service (noun)	airpost (noun)
the system whereby messages are transmitted via the post office	a system of conveying mail by aircraft