The answer is the front window. Dogs that have access to look out the front window of the house are much more reactive at people walking by and coming to the house. And every single day they get rewarded for their behavior.
Lets look at what happens from the dogs perspective.
1)Mailman approaches
Your dog thinks: Here comes this strange looking guy in a uniform walking right up to my house. Im going to bark my head off to scare him away.
2)Mailman drops mail in box and walks away.
Your dog thinks: Thats right, you better get out of here. Ha I scared that punk away. Yay for me!
Your dog is rewarded every single time the mailman leaves. Your dog thinks his reaction is what sent him on his way. The same holds true for anyone walking by (or walking a dog by) they walk in front of your house, your dog reacts, and they leave. Fido thinks hes doing a fine job protecting the perimeter.
This is why treating barking at people and dogs as they walk by the house is impossible. For us to successfully work on this issue we would need to control the person coming toward the house. The person walking by would have to stop when the dog reacts and wait while we work with the dog to get them back to a calm state of mind, and then have them continue on their way. Just try getting the mailman to do that.
Theres just no way to control the environment, so it is impossible to work on. The best thing to do, if possible, is to not allow your dog access to the front window. I know it sounds like a cop out of training but there really is no way to properly address this behavior. And dogs that react at people and dogs at the window are more likely to carry this habit outside and react on leash.
Sometimes you have to accept that the situation is uncontrollable and instead of solving the issue, we concentrate on trying to manage it. So if you can, close up the shades or keep your dog away from the lookout points. His barking is not going to get better and your mailman is thinking of throwing that Ikea catalog at the window.