Preheat oven to 350 degrees F. Lightly spray or butter a 9 by 4-inch loaf pan and set aside.
In a medium mixing bowl, whisk together the flour, poppy seeds, baking powder, baking soda and salt until fully combined. Set aside.
In the bowl of an electric mixer, add the sugar and lemon zest and rub together with your fingertips until sugar becomes moist. Attach the paddle attachment and add the butter to the sugar mixture. Mix on medium until butter is soft and fluffy, approximately 2 minutes. If necessary, occasionally scrape sides of the bowl.
Mix in the eggs, one at a time, mixing between each addition. Add the vanilla and mix once more.
In a small microwave safe bowl or container, mix together the milk and sour cream. Place in the microwave and heat until mixture is no longer cold (room temperature is the ideal temperature here).
Finally, working in 3 separate batches and beginning and ending with the flour mixture, add 1/3 of the flour mixture to the butter/sugar mixture, alternating with half the milk mixture and 1 Tablespoon lemon juice, mixing until just combined after each addition.
Pour batter into prepared loaf pan and bake at 350 degrees F for approximately 45-55 minutes. To prevent the top from burning, tent the top with foil approximately 30 minutes into baking, being careful not to touch the loaf with the foil. Bake until toothpick inserted into the center comes out clean. Remove from oven and allow loaf to cool for 5 minutes before inverting onto a wire rack and removing from pan. Cut loaf into slices and store any remaining pieces in an air-tight container to maintain freshness.