Ever wonder why so many dogs seem to truly, sincerely HATE the mailman? Besides the edging-on-creepish outfits (scary uniform-style shorts, spooky wheely-contraption bag as accessory...), many freaked-out pups are simply reacting to their natural doggy drives.
It's really quite simple (and comical, actually!) Most dogs don't understand that it's the mailman's JOB to come to the house everyday, drop off the mail, and then... leave . Unless he's been taught otherwise, a dog is thinking: "Here he comes... that relentless bad guy is going to try to get into my house AGAIN... if he thinks he's going to get in this time, he's wrong! I'm going to bark and growl until I scare him away!" After the postman leaves, your dog feels quite pleased, because he believes he has successfully thwarted an evil-doer's dastardly plans!
Trainers call this self-rewarding behavior. Instead of treats, your dog's repeated behavior has been rewarded with that warm, glowy inner feeling of accomplishment called... self-esteem (it's as good as praise, and almost as great as chicken!)
Understanding why a dog does things is superimportant. Knowing that the motivation is well-intentioned often softens our feelings of annoyance, and it can also give us greater insight on how to attempt to remedy a situation. As always, each dog is an individual, and there are different techniques to fix this problem depending on your dog's age, breed, and temperment. One often highly-effective technique is to change your dog's perspective of the mailman from intruder to... friendly human treat dispenser. You'll basically be putting the TREAT in peace treaty :)
Unfortunately, a lot of postal carriers have had really negative experiences with dogs! But (assuming your dog is not aggressive), if you approach your postal carrier with your dog trainer, your mailman may be more willing to meet your dog (on-leash, of course.) He can then help you start the desensitization process: by silently tossing your dog an ultrasupercovetedfavorite treat. He or she may be willing to repeat this each day he or she drops off the mail (make sure that your dog only evereverever gets this supertreat from the mail carrier, and that when it happens, your dog is always supervised, on-leash.) Pair the treat-giving with lots of calm, loving praise from you, and soon mail delivery will be like Doggy Christmas.
If your dog begins to become overstimulated and starts barking, respond silently, unemotionally and immediately... away you go to another location: No treat.
Over time, most dogs will come to associate calm behavior around the mailman with yummy goodness. You might even begin to find your dog waiting happily at the front door in anticipation of your postal carrier's delicious Santaesque arrival! After lots of practice (we might be thinking weeks not days; although this will greatly depend upon your postal carrier's willingness to participate), the mail carrier can stop tossing out treats, and when he or she arrives, you can give your dog the treats, instead. Once you observe that the positive association is really solid, you can gradually begin to phase the treats out completely.