We haven't had a problem with stolen mailboxes, we have one of those plastic barn mailboxes and the plows bump out at mailboxes because its the homeowners responsibility to mow, plow, shovel around the mailbox for the mail car.
my uncle had his knocked off so many times he build a steel box to put the mail box inside and welded it to a nice piece of steel he cemented into the ground. Never lost it again. It could be painted nice and flowers grown up around it.