Summarize:

Alarm barking is any pattern of barking at perceived intruders. While barking at a real intruder is useful and may save a person's life, barking at perceived intruders like mail carriers, parcel deliverers, or even just neighbors passing by the property can be annoying and troublesome.  Alarm barking does not always require a visual confirmation of the perceived intruder. Some dogs may engage in alarm barking simply from hearing a car door outside or hearing voices on the sidewalk.  Alarm barking is often accompanied by a slight lunge or pounce forward (one to two inches) with each bark. The best way to quell alarm barking is by teaching your dog to be quiet on command. Like any training, this will most likely be a time-consuming process that requires patience and consistency. But if you're willing to put in the time and effort, even the most territorial dog will learn to behave better.  When your dog starts engaging in alarm barking, hold up a treat after three or four barks. This will get her attention and will most likely distract her from the perceived intruder.  Wait until she stops barking. Simply be patient and continue to hold up the treat.  Once your dog stops barking, say "quiet" in a calm but stern voice and give her the treat.  Repeat this process until your dog learns to associate the word "quiet" with her silence. Once your dog has done this successfully on 10 or more occasions, you can begin giving the quiet command without showing her a treat. If she still complies with your command, then give her a treat. If she does not, you may need to show her the treat for several more training sessions.  Eventually your dog will learn to be quiet on command without getting a treat. Even after you've reached this stage of training, however, you should still give your dog verbal praise when she stops barking. Once your dog has learned the quiet command in training sessions, you'll need to apply the quiet command to real-world scenarios. You can do this by having a friend slam a car door in front of your house, rattle your mailbox, or approach your front door.  Have a treat ready every time your friend comes to the door. Even if you've passed the point of giving treats during regular training, you may need to use treats for applied training sessions involving an actual perceived intruder.  When you have someone come to the door pretending to be the mailman, it's imperative that your friend does not leave the porch until your dog is quiet. If he leaves while she is still barking, she may come to think that it was her barking which drove him away.
Recognize alarm barking. Teach your dog the quiet command. Apply the quiet command.