What is the proper setback from the curb for a mailbox?
6-8 inches from the curb according to the official USPS website.

Of course, this refers to the location of the door relative to the front face of the curb. If the plow is taking out the post, rather than the box itself, you could install a post that is angled, rather than vertical. My dad would cement a 6-foot steel fence post 5 feet into the ground in front of the mailbox post. "That'll show 'em!"