I have a theory. Dogs are territorial. People have different smells and hormones. When the same person mailman or even a different mailman comes by the dog's yard and just walks at a pace long enough to just stuff mail in the box, the dog does not have the opportunity to really smell out that person. It is like a tease for the dog every time. Like,"Hi, I am a stranger in your area, I am going to touch something of yours, and walk away quickly." I would be somewhat annoyed to, lol.
That's just a stereotype. My dog loves the mailman. He often gives my dog bones when he come up to his truck. Not to mention all the yummy packages my dog gets to rip to shreds when I'm not there to take them!