There is more to consider than just the size or age of the dog.

Small dogs are often more aggressive and may be more prone to bite. Why should the mailman have to be bitten by a wiener dog at all? And when/if he is bitten, then what comes next? Does he need to verify that the dog has had it's shots. How does he do that? Is there any guarantee that the dog is in its owner's yard? I've had dogs that aren't mine in my front yard all the time.

Older dogs can also be a little more aggressive and less patient. So the dog seems to be moving slowly. What guarantees are there that it won't lunge and bite? And again, why should the mailman be required to take on that risk?

My brother was bitten by an "old" golden retriever that had never bitten anyone before. He pulled his arm away when that happened, which resulted in the dog's teeth ripping down his arm. He still has a 2.5 inch scar on his forearm from where he had to go get stitched closed.

All dogs are safe and don't bite, until they do.

I totally get what you're saying. But the mailman is just hurrying to get his job done. He shouldn't have to stop and evaluate how dangerous a dog may or may not be as he goes - if you want your mail delivered keep your dog secured. Sure - things happen and animals get out. But it should be rare. And it shouldn't be the mailman's responsibility to be an animal behavior specialist and know if he's about to get bit or not.