I had a friend do the whole concrete mailbox thing 20 some odd years ago. Dangerous or not, it was still funny when he painted little baseball bat "kills" on the side every time he found a new scuff or broken bat.

Also, I'm no expert in mailbox law, but all the ones in my neighborhood and the surrounding neighborhoods are encased in brick. They'll crumble if you hit them with your car, but no one is trying to take a bat to them.