Since we have the occasional snow plow come through along with the bored teenager with a baseball bat, here's what I did to protect my mailbox for my extremely rural cabin in the woods.
First thing I did was drill a bunch of lag bolts into the bottom twelve inches of the mailbox post. I put the end of the wooden post in an empty five gallon bucket and filled the rest with cement. The lag bolts prevent someone from pulling the mailbox post out of the bucket and stealing it. Because the post is not attached to the ground, if it gets clipped by the snow plow it will fall over backwards. The bucket is high enough where a car bumper will also just knock it backwards while causing lots of damage to their vehicle. Haha!
The next thing I did was to place two green metal fence posts normally used with barbed wire level with the front of the box. This will protect the box from baseball bats and allow one to put some additional reflectors to warn drivers. I then wired the mailbox to the metal posts to prevent someone grabbing the whole bucket and box, and when it falls backwards the wiring will catch it before the box hits the ground. It's survived several years out here while my neighbor has replaced his box twice.
Iron Noder 2017