At some point you will have two characters talking–or exchanging letters–and they will mention a third person–or even a bunch of people. And that’s when you must be careful. In your mind it might be perfectly reasonable that one character would say, “Paul gave me your letters when he came up to camp last week.” Because in your authorial omniscience, you know that Paul is a good friend of the speaker and that the person he is writing/talking to would know this. But if Paul is just a vehicle for delivering the letters, he might not have appeared before, and might not be mentioned again. In that case, the reader is likely to be distracted–wondering who Paul is and whether he is important to the story.
Bottom line: if you name a person, make sure the reader can put that person in context. E.g., in the example above, you might say, “You remember my friend Paul? He brought your letters…” Alternatively, work around the whole name thing by naming the relationship–as in, “My brother brought your letters…”