ItemStack equipped = getEquippedItem(player);
if (equipped == null)
return null;
if (equipped.getItem() instanceof ItemLetter)
return new GuiLetter(player, new LetterInventory(equipped));
else
return null;
case MailboxGUI:
return new GuiMailbox(player.inventory, (MachineMailbox) getTileForestry(world, x, y, z));