postman vs letter carrier

postman and letter carrier both are nouns.

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

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

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

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