Preheat oven to 375ºF. Line a 12-cup muffin tin with paper liners or lightly grease with cooking spray; set aside. In a large bowl, combine flour and salt; whisk. In a medium bowl, combine sugar, milk, butter, poppy seeds, eggs, and lemon zest; whisk until well blended. Pour the wet ingredients in the dry ingredients.
In a small bowl, combine baking powder and lemon juice, stir then pour in the dry ingredients. Stir until batter is just combined. Divide batter evenly among muffin molds. Transfer to the preheat oven and bake for 18 minutes or until a cake tester inserted in the center comes out almost clean. Cool 5 minutes before transferring onto a wire rack.
Note: Combine baking powder with lemon juice helps to make muffins fluffier and rise better.