Bring a large pot of water to a boil. Add the macaroni and cook for 8-9 minutes, until al dente. Drain macaroni and set aside.

In the same pot over medium low heat, add the butter. Once the butter is melted, add the flour and whisk until bubbly, about 1 minute. Add the salt, pepper and dry mustard and cook another 1 minute. Pour in the milk and raise the heat to high, stirring constantly.

Bring the milk to a boil and then reduce to low heat. Stir in the cheese and pimentos until the mixture is creamy. Remove from heat and stir in the macaroni. Serve warm.