All of my dogs have met the mailman. He knows them by name. As long as it isn't another mail person filling in, I don't have a problem. You might want to talk to the mailman. Ask him if he could meet your dog and give him a treat. See if he can nicely say the dog's name as he is walking up so that the dog knows it's him.

I work for the phone company...There has been many times where our techs have been attacked by pets that broke though doors, windows, etc... Even the sweetest family pets may want to protect their family when someone is on their property. It's natural.

I don't know your doggy personally, but it might not be a bad idea to crate him if you can't work things out.