Mail box box
A clever play on words!
A "mail box" is a container used to receive and store mail, typically located at the entrance of a residence or business.
But, I'm assuming you're referring to a "box within a box" scenario, where you have a box that contains another box, which is often used to describe a nested or layered structure.
In this case, the outer box is the "mail box" that contains the inner box, which could be used to store smaller items or packages.
Am I correct in my understanding?