Cook macaroni according to package directions using salted water. Drain and run cool water over the pasta until it is cooled.
In another bowl stir together mayo, mustard, vinegar, pepper, sugar, salt, garlic, and onion powder. Add the onion, celery, and bell pepper. Mix well.
In a large bowl, add the pasta and about half of the dressing and toss to coat. Cover and refrigerate the pasta and remaining dressing for at least 2 hours.
When ready to serve, add remaining dressing and toss to coat. Add additional salt and pepper to taste.
