letter vs mail

letter and mail both are nouns.

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

letter (noun)	mail (noun)
a written message addressed to a person or organization	(Middle Ages) flexible armor made of interlinked metal rings
the conventional characters of the alphabet used to represent speech	a conveyance that transports the letters and packages that are conveyed by the postal system
an award earned by participation in a school sport	the system whereby messages are transmitted via the post office
a strictly literal interpretation (as distinct from the intention)	the bags of letters and packages that are transported by the postal service
owner who lets another person use something (housing usually) for hire	any particular collection of letters or packages that is delivered

letter (verb)	mail (verb)
mark letters on or mark with letters	cause to be directed or transmitted to another place
set down or print with letters	send via the postal service
win an athletic letter