Anyhow, they are not mail boxes, they have been reconfigured to be company issued personal EFB storage portals!
Pre flight. Retrieve iPad EFB from said portal 5secs before report for duty and switch it on as you approach the sign on point.
Post flight. Turn it off and place in personal EFB storage portal (formerly known as FOP crew mail box).
Surely this is the safest and most risk free storage of an important operational device?