airpost vs mail service

airpost and mail service both are nouns.

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

As nouns, mail service is a hypernym of airpost; that is, mail service is a word with a broader meaning than airpost:

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

Other hypernyms of airpost include mail, post, postal service.

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