snail mail vs mail service

snail mail and mail service both are nouns.

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

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

snail mail: any mail that is physically delivered by the postal service
mail service: the system whereby messages are transmitted via the post office

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

snail mail (noun)	mail service (noun)
any mail that is physically delivered by the postal service	the system whereby messages are transmitted via the post office