I had a removable mail box & it got stolen. Sheriff brought it back after I had already replaced it. They had found about a dozen mail boxes dumped in one place.
However, my mailbox frequently got damaged and I suspected my neighbor across the road was just careless, hitting it with his truck or whatever. Kind of a low life. I could tell someone from his place had backed into my mailbox at least once.
So I got some diamond plate, had it bent to cover the mailbox with a 1" or so gap between. Set it on a 6" diameter steel post, mounted in a 2', gravel filled hole, located about 6 ft. off the road, adding gravel to widen the shoulder. Post office folks like it because they are off the road when the deliver the mail. My mailbox hasn't been scratched since then.
I have heard about some folks who mount their steel post on a coil spring so it will give if hit by a car to reduce the chance of injuries.