An error causing unread letters to be hidden and inaccessible has now been fixed. 

The mailbox located in front of the player's house was intended to bear similarities to the mailbox in Animal Crossing. Villagers/NPCs would send the player updates based on their interactions. Unfortunately, one letter in particular wouldn't display the "unread mail" symbol and would prevent any other letters from being opened.

The letter in question is from Rosie, the Tea House lady, asking for a koi fish. If you, prior to the update, interacted with Rosie and her friend more than 2 or 3 times, Rosie will send you an "invisible" letter that prevents opening any other letters from that point on. If you want to access your mail, please update the game. Thank you and sorry for the inconvenience!