Ding-resistant mailboxes
As a mail carrier myself, I can tell you that the better one-piece mailboxes like the Step 2 (
www.step2.com ) mailmaster series sold at most home centers are pretty much impervious to neighborhood vandals. It generally takes a direct hit from a car or snowplow to do one of these in. At around $40-50 for the whole shebang (post/box) this is about the best solution. Some models even have a provision for an integrated newspaper recepticle as well. They look nice and neat, come in many non-fade colors, many styles, don't rust, and take most abuse in stride.
Yes, many communities/counties have restrictions on placing solid objects near the right of way, and this precludes many of the stronger mailbox or post designs. A couple of neighbors here in our rural area were told to cease and desist from building brick enclosures for thier mailboxes. We live on a County road, and that carries the restriction. If you really think about it, many of the highway sign and lightposts alongside the road are made to be break-away in the case they're hit by cars.
As for making the box itself heavy, like from 1/4" plate steel or similar, you might run into a problem if someone accidentally runs off the road, hits the box, and that 40-pound chunk of steel goes through the windshield and kills someone. Those bottom-feeding lawyers will try every trick in the book to get big $$$$ out of you if this were to happen. I wouldn't be at all surprised if the USPS itself has a regulation regarding the max weight of a residential curbside mailbox for this very reason. I'll have to check into that.
I have seen some people put guards in front of their mailboxes. Over in heavy snowfall areas, on the major roads where the snow plows are really moving, that thrown snow can take out a box rather easily. They use a couple of 1-1/2"-2" galanized metal pipes embedded in the concrete and on front and back corners of the traffic side of the mailbox, connected at the top with elbows and another short section of pipe. It looks like a big inverted 'U' . In winter they fasten a piece of plywood between the two uprights and that breaks up or deflects the snow thrown from the plows. A side benefit is the mailbox now has a steel pipe guard in front of it to discourage baseball-bat wielding punks in cars.