mail service vs communication

mail service and communication both are nouns.

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

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

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

Other hypernyms of mail service include communicating.

mail service (noun)	communication (noun)
the system whereby messages are transmitted via the post office	something that is communicated by or to or between people or groups
	the activity of communicating; the activity of conveying information
	a connection allowing access between persons or places