mailman vs letter carrier

mailman and letter carrier both are nouns.

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

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

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

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