mail carrier vs letter carrier

mail carrier and letter carrier both are nouns.

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

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

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

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