Mailbox is a custom item made especially for Blackblock.
The Mailbox is an inventory block where players can leave messages/items for each other. If a mailbox has an owner, only the owner (or the person who originally put in an item) will be able to take the item back out.
The mailbox has 24 inventory slots available for general items, excluding other mailboxes.
The 25th slot on the right (which looks like a stamp) is where players can put their Passport. When this card has been inserted, the mailbox will be linked to their account and only they will be able to take items out.
Items in the mailbox will always show the player that last moved the item, and when they last did so.
Be careful, because once you take the item out that data will be gone.