Finely grate 1 teaspoon lemon rind; squeeze 3 tablespoons juice from lemon over a medium bowl. Add rind, mayonnaise, and pepper to bowl; stir well. Let stand 5 minutes. Add salmon to bowl; stir until well blended.
Layer cucumber evenly over 4 bread slices; top each with about 1/2 cup salmon mixture. Top with remaining 4 bread slices.