mail service vs communicating

mail service and communicating both are nouns.

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

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

mail service: the system whereby messages are transmitted via the post office
communicating: the activity of communicating; the activity of conveying information

Other hypernyms of mail service include communication.

mail service (noun)	communicating (noun)
the system whereby messages are transmitted via the post office	the activity of communicating; the activity of conveying information