An aggressive dog is always potentially dangerous. But if your dog misbehaves in a non-threatening manner around a new baby, the postman or anyone else, you may be able to modify its behavior with positive reinforcement. You might try showering it with attention and treats when the disliked person is present. The dog may gradually begin to enjoy the person's presence.
Are you trying to stop a dogfight? Don't yell at the dogs; make a loud noise, press your car horn, ring a bell, or beat on a frying pan. If this fails, douse the combatants with a hose or with buckets of water.