I did interact with all the mailboxes, or at least I thought I had. I just always got the message about there being nothing in them. I have a save before the last boss, so I can go back and check that out again, though.
I'd love to see you add or expand where possible, cause it was just such an enjoyable game and the story was engaging. I liked visiting the different worlds and seeing what befell them. I'm just so impressed with the amount of work you got done!