In a large bowl, beat the egg whites until stiff peaks form and set aside. In a separate bowl, beat the egg yolks. Add the lemon peel, juice and milk and mix well, then add the sugar, flour, and salt and beat until smooth. Gently fold in the egg whites.
Carefully pour into an ungreased 6x9" casserole dish. Place the dish into a larger pan of very hot water (1 inch deep) and bake for about 50 minutes, or until the cake just jiggles slightly. Allow to cool for about 15 minutes before serving.