"I received a letter from a person named Dupont" would be fine, although probably "from somebody named Dupont" would be more likely.
More formally someone might say, "I received a letter from a Mr./Ms. Dupont," or "from a certain Mr./Ms. Dupont".
As for the difference between "named" and "called", in this case I think it would certainly be "named", since you're giving the person's actual name. But if it was a letter from someone who used their nickname, then you would probably say "called": e.g. I got a letter from someone called "moon0719".