Dogs are naturally protective. They see visitors as one of two things: friends or intruders. From a dog’s point of view, a postal worker/mailman who comes most days carrying packages with strange smells, and who is not introduced, is a trespasser! Their instinct kicks in and they start barking to eliminate the ‘threat’. Once the postal worker leaves, your dog feels relieved and like they have accomplished their task. This reinforces their behavior and they continue to repeat it.