In a large bowl, whisk together cornmeal, rye, whole-wheat and all-purpose flours, salt, and baking powder and soda; set aside. In another bowl, whisk together molasses, buttermilk, and egg until smooth; pour over dry ingredients, and stir with a spoon until just combined. Transfer to a cleaned and greased 13-oz. metal coffee can, and cover can with a piece of foil; place in the center of a 6-qt. saucepan. Pour 6 cups water into pot around can, and bring to a boil over high heat.