Do you mean the letter in the in-game mail in the game itself? If so, there's no way to return it. Was there anything in there that was important to you?
Maybe it's not that important, but I want to keep it.
I think it's not so hard for backend programmers to just recover one letter, after all, I've only received one letter from the Booty Heroes team in the 'Breaking News 04.08.2023', which I received when the Journey was reset, and I'm sure you must have a record.