Beware of the dog signs?

You would have to consult a lawyer to be sure, but here in the US (California), if you put up a "Beware of Dog" sign, the line of thought is that you KNEW your dog was dangerous (and the fact that the person had no business being IN your yard, was warned, and IGNORED the warning completely disregarded). So much for justice. About the mail, get an outdoor mailbox, lol.