Mailbox construction is a double edge sword.
USPS wants the indestructible kind. The only regulations USPS has pertaining to mailboxes is set back from the street, height & size.
SHA is a different story. The location the USPS wants the box to be is in SHA territory. SHA requires that the pole be a break-away type- wood, steel that will bend, fiberglass. They advised that if I was to install a non-break-away type of pole that I would be held liable in the event of an accident.
I liked the idea of a heavy steel mailbox but I didn't like the idea of a heavy steel mailbox flying thru the air when they wrecked into it..
After pondering the liability issue, I ended up getting a PO box..
This was about 9 years ago..