This is a big problem for many people in uniforms. Dogs see them as strange and feel threatened. The best way to stop the barking is by using classical counterconditioning. I have a hub about the subject "How to help a dog that is afraid of fireworks" and Alexadry also has a hub about the subject. The method will require the assistance of the mail carrier, UPS person, meter reader, etc. When the person approaches the dog they need to toss a special treat, something the dog does not normally get. Tell them they can not hand it to the dog and get into the animal´s comfort zone. Just toss it and walk away. The amount of time will vary for each dog but if the person does it every day the dog will look on that person as a distributor of treats, not an invader. Again, it is important that they not try to hand the treat to the dog, just toss it from a distance.