We have been the recipients of games of "mail box baseball" about 6 times in the 3 years we've lived here. I talked to the folks at the post office and they recommended 2 metal posts driven outside the mailbox highly visible, but protecting the box itself. When I lived in the country in Missouri, I bought a big metal Milk can, put the mailbox post in it, then put rocks in it to hold it upright. I leveled a spot on the side of the road to sit it flat, then when I got it in position, I poured a couple of bags of quickcrete powder in with the rocks, then added water so it would set up. if a car would hit the can/box it would fall over, but it would also do some dandy damage in the meantime.
I also painted it a nice reflective silver so it's very easily seen. I never had any problems there with that being hit.