We live out in the country and while we've never had our mailbox vandalized (I did chase down some kids I caught doing it to other's mailboxes in our area 20 years ago), we do have a problem with snowplows in the winter. Our next door neighbors typically replace theirs every year or two because a snowplow takes it out.
We've been very lucky with our so far though. It is just a relatively inexpensive (around $10-15 probably, it was left by the previous owners) plastic mailbox mounted on a 4" round post. When the snowplow hits it, the plastic deforms, pops the lid off its hinges, and the plastic mailbox retains its shape. I just keep an eye out for when the snowplows run, and go out and dig my mailbox lid out of a snowbank and snap it back into place.
I've though of putting something very heavy-duty in place, but I would be worried about the liability of damaging a snowplow or having someone accidentally hit it, we live on a busy state highway with a speed limit of 55 mph and traffic is usually moving at 65+ mph. As long as our cheap little plastic mailbox keeps bouncing back into place, I'll stick with it.