Was there a system in place? Would it be possible for them to get a reply back? How did this work? Links or book recommendations welcome. Information even semi related welcome.
Something similar to this was asked once before & u/intangible-tangerine gave an excellent answer.
As far as the explorers and international mail, as far as I know there was no real “system” in place. It was more just a matter of hand the letter to someone you trust on their voyage back, and say “here deliver this to Thomas, please”.