
Probably the greatest hazard to letter carriers are dogs. I can be at Wal-Mart, with a dog in a car, and they do not bark at people. Soon as I walk by in my uniform, they bark. There is something about a mailman that interests dogs.
The most common remark I hear is "He doesn't bite." Many a mailman has been bit by dogs that don't bite. I'm always wary of a dog until I've observed them. It may be true that he hasn't bit anyone yet, but there is always a first time.
Breeds are sometimes an indication of aggressiveness, but not always. I've met boxers and pit bulls that are very friendly, and I've been attacked by a toy terrier who only weighed 4 pounds.
There's not much to say about dogs, other than keep them contained. Keep them on a chain or in the back yard fence. Letter carriers frequently skip houses with loose dogs, and in extreme cases entire streets may have to be skipped due to an aggressive dog that is loose.
Here is a helpful web page on improving the mailman/dog relationship.