letter carrier vs mail carrier

letter carrier and mail carrier both are nouns.

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

As nouns, letter carrier and mail carrier are synonyms defined as:

Other synonyms of letter carrier include carrier, mailman, postman.

letter carrier (noun)	mail carrier (noun)
a man who delivers the mail	a man who delivers the mail