Can I use a steel post for my mailbox?

Installing the Mailbox Post The best mailbox supports are stable but bend or fall away if a car hits them. The Federal Highway Administration recommends: A 4″ x 4″ wooden support or a 2″-diameter standard steel or aluminum pipe.

Is it illegal to make your mailbox indestructible?

The answer is no. You don’t owe a duty of care to someone to pad your mailbox so it is soft for them when they maliciously try to destroy it.