RFD vs mail service

RFD and mail service both are nouns.

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

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

RFD: free government delivery of mail in outlying country areas
mail service: the system whereby messages are transmitted via the post office

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

RFD (noun)	mail service (noun)
free government delivery of mail in outlying country areas	the system whereby messages are transmitted via the post office