Dog owners need to take responsibility. Our dog used to jump up and snatch the post. I started training him to stop doing it, but to be safe I put a cage behind the letter box to protect anyone. Even a sign is not good enough, it could be missed.
Dogs can be dangerous animals. You have to be responsible when owning one. If my dog hurt someone it would be my fault, no doubt. I've let that person down.
Postmen are told NOT to put fingers in letterboxes, but it's easy to forget sometimes and some dogs sit silently waiting to attack with no warning.
In my experience, some dog owners usually blame the person for doing wrong and not themsleves or their pet. They get mad when postmen refuse to deliver to their property due to a dangerous dog. That's stupid.