Cook macaroni in a large pot of water with the tablespoon salt until just tender. Reserve 1/2 cup pasta water, then drain.
While the pasta cooks, melt butter in a saucepan over medium heat. Whisk in flour for 1 minute, then gradually add milk, mustard, sauce salt and pepper.
Simmer for 3 to 4 minutes until thickened. Stir in cream cheese until smooth, then remove from the heat and fold in cheddar.
Fold in drained pimentos and mayonnaise, then add macaroni. Loosen with reserved pasta water a tablespoon at a time if necessary.
Let stand for 1 minute and serve, keeping the sauce below a boil after the final cheese addition.