Preheat the oven to 350 F (180 C).
Butter and flour two 8×5-inch (20 cm x 12.5 cm) bread molds.
Sift together the flour and baking powder and set aside.
Beat with an electric mixer the sugar, lemon zest, and eggs until the mixture is white and has doubled in volume.
Add the sour cream, salt, rum, and lemon juice and mix.
Add the flour and melted butter alternately and mix.
The dough will be thick and flowing.
Divide the dough evenly between the two prepared molds and bake for 50 to 55 minutes, or until a toothpick inserted in the center comes out clean.
While the bread is baking, prepare the syrup.
Boil together the sugar and water, stirring until the sugar is dissolved.
Remove from heat and add the lemon juice.
Let the bread cool in the molds, then pour the warm lemon syrup over it.
Let it cool completely.
Serve sliced, accompanied by lemon curd or a red fruit jam if desired.
The bread can be stored, covered, at room temperature or refrigerated for 5 to 7 days. It can be frozen, well wrapped, for 6 months.