snail mail vs postal service

snail mail and postal service both are nouns.

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

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

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

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

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