Block is not protected by itself, place into your claim. Two upper slots can be accessed by anyone, by button you can send them into closed slots which are available only for owner.
If there is something in mailbox flag will be up. If you hover over button there should be owner's name.

Shift click doesn't work, right works as it instead.

Additional functionality addition will need Mudwall Senators' decision.

Models provided by Morgen and Ragolution.