We had the same problem when I was a kid growing up. Dad replaced the wood post with a round steel one. Slid another lager pole over it, and welded on the arm for the mailbox. Drilled a hole through both posts and installed a long, soft steel bolt.
So when the snow plow driver came along and hit it, the bolt would shear off, send the mailbox into a clockwise orbit, and smack the truck in the ass. Replace the bolt and the cycle would start all over.