I built a killer mailbox. The box itself weighed 30# and I screwed it to a steel plate welded to a water pipe flange and a piece of water pipe which set inside a galvanized fence pipe set into concrete. I designed it so it could spin if hit.

I came home one day and the fence pipe was wrapped around itself like a twizzle stick and the mailbox was just.. gone. I went up and down the road looking for it. Neighbor said it made a helluva noise when it was hit.

Ever since then, fuck it. 4x4 post and a $12 plastic box.

I did move the post back away from the road as much as possible and that has reduced the hits by a lot. They have to be fully in the ditch to get to it and they're going airborne due to the culvert if they do.