Barking is quite self-rewarding behaviour - some dogs just love the sound of their own voices

The thing with the postie is, that to the dog he is making the postie go away. It doesn't occur to him that the postie would go away anyway Same with dogs which bark at thunder.

Does he bark when he's left alone, while you're out etc?

Personally I like my dogs to bark a couple of times - it a) Alerts me to someone at the door (which I don't usually hear) and b) It alerts potential intruders that there are dogs in the house.

It sounds as if the best thing you could do is teach a "speak" on command, and conversely a "quiet" command, so that after a couple of woofs you can turn it off