It's a fact of life: Dogs bark. It's unrealistic to train a dog to stop barking altogether, and why would you want to? Barking dogs are terrific crime deterrents. However, you can put Rover's alarm barking under your control so it's not excessive. Fearful alarm barking means Rover sounds off at any noise. He's not only triggered by the mailman or doorbell, he barks at everything. Training him to decrease barking requires you to teach him that barking is okay until you say "Quiet." This way you'll retain some barking and manage it on your terms, not his.
Until you have retrained Rover to decrease his barking, he should be confined to a place in your home away from noise. Don't isolate him. Your dog should be in an area that's close to his humans, but relatively sound-proof. Close the drapes to prevent him from barking at pedestrians, mask other "trigger" noises (like a school bus or delivery truck) with the radio, even temporarily disconnect the doorbell to prevent a barkathon. Do this while you're training him to set him up for success.
After Rover barks two to three times, praise him for sounding the alarm, then tell him "Quiet." Wiggle a tasty treat (the good stuff like boiled chicken, ham chunks or liver bits) in front of his nose, but don't let him have it yet. Prevent him from snatching it away by putting the treat in a closed fist. Most dogs immediately stop barking because they can't bark and sniff simultaneously.
Delay giving your dog the treat for three to five seconds, no more. If he is silent, lavish praise on him and reward with the treat. If he sounds off, dramatically whisk away the treat and put it in your pocket. Wait a few seconds and try again.
Keep training sessions short and positive, under five minutes each. Work with your dog every day, several times a day if possible. Work with him in various rooms in your house so he can learn to generalize the behavior. Keep your expectation of "Quiet" to three to five seconds in the first week of training. If he barks before the time limit is up, lower your expectation and decrease the time requirement by a few seconds.
Once Rover is quiet for up to five seconds, increase the time he's required to be silent by a few seconds, then reward. By adding time gradually to the quiet command, you teach your dog restraint, barking will decrease and your dog learns that "Quiet" is fun and rewarding.