Heat oven to 300 degrees and thoroughly grease a 9x5 inch loaf pan. Set aside.
In a medium bowl, mix together the flour, baking powder and salt. Set aside.
Using a handheld mixer or stand mixer, beat together the melted butter and sugar until it turns a pale light color, about 3 minutes. Slowly add the eggs to the mixture with the mixer on low speed. Add the lemon juice, zest, sour cream and poppy seeds and mix until combined, about 1 minute.
Keeping the mixer on low, add half the milk and half the flour until combined. Add the remaining milk and the rest of the flour and gently fold using a rubber spatula until just combined.
Transfer batter to the loaf pan, it's okay if it doesn't fill all the way. Bake in preheated oven for 1 hour 10 minutes to 1 hour and 20 minutes. Start checking it at 1 hour and 10 minutes to see if the top middle is set. Check for doneness by inserting a toothpick in the center. If it comes out clean it's done. Cool in pan for 20 minutes then transfer to wiring cooling rack.
While the bread is cooling in the pan, add the powdered sugar to a bowl with the water and mix well. Pour the icing over the bread while on the wire rack, with a piece of parchment underneath. Allow to cool completely. Store in an airtight container for up to a week or in the freezer for up to three months.