mail vs RFD

mail and RFD both are nouns.

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

As nouns, RFD is a hyponym of mail; that is, RFD is a word with a more specific, narrower meaning than mail:

Other hyponyms of mail include airmail, airpost, snail mail, rural free delivery.

mail (noun)	RFD (noun)
(Middle Ages) flexible armor made of interlinked metal rings	free government delivery of mail in outlying country areas
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

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