RFD vs mail

RFD and mail both are nouns.

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

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

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

RFD (noun)	mail (noun)
free government delivery of mail in outlying country areas	(Middle Ages) flexible armor made of interlinked metal rings
	a conveyance that transports the letters and packages that are conveyed by the postal system
	the system whereby messages are transmitted via the post office
	the bags of letters and packages that are transported by the postal service
	any particular collection of letters or packages that is delivered

RFD (verb)	mail (verb)
	cause to be directed or transmitted to another place
	send via the postal service