postal service vs mail service

postal service and mail service both are nouns.

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

As nouns, postal service and mail service are synonyms defined as:

postal service and mail service: the system whereby messages are transmitted via the post office

Other synonyms of postal service include mail, post.

postal service (noun)	mail service (noun)
the system whereby messages are transmitted via the post office	the system whereby messages are transmitted via the post office