Here is my .02 cents. I read electric meters for 12 years and the dog trainers told us that day after day the mail man comes on to the dogs territory. the dogs barks and barks and the bad man leaves. It gets reinforced everyday so the dog thinks that every time he comes the dog chase's him off. Same with UPS and all others.
Everybody hates the mailman, but this is pretty much the truth, the dogs get used to barking and the offender goes away. plus the fact that dogs see in black and white, and it looks strange to them to see someone all in one color.