Preheat oven to 375 degrees F. Lightly grease a 13x9-inch baking dish or spray with nonstick spray.
In a medium bowl, combine 2 1/2 cups flour, baking powder, and salt. Set aside.
In a large bowl, beat together the mayonnaise, granulated sugar, and vanilla with an electric mixer on medium speed until combined. Add eggs and beat on low until combined.
While beating on low, add half the flour mixture to the large bowl, then half the milk, then the remaining flour, then milk. Pour batter into prepared pan.
Top cake batter with an even layer of peach slices, pressing the peaches slightly into batter.
In another large bowl prepare crumb topping by mixing together 3 cups flour, brown sugar, and cinnamon until combined. Pour cooled melted butter on top. With a large spoon or rubber spatula, gently lift the flour mixture from the bottom of the bowl to the top. Repeat until crumbs form. (Note: Do not mix vigorously or overmix because mixture will turn to mush. It's ok if not all the flour is incorporated.) Sprinkle crumb topping evenly over peaches and cake batter.
Bake cake in top third of oven at 375 degrees F for about 35-45 minutes or until toothpick inserted comes out clean. If the crumb topping starts to darken before cake is done baking, cover the top loosely with aluminum foil and continue baking.
SNAPPY TIPS: I like to bake the cake towards the top of the oven so the crumb topping gets browned and slightly crunchy.SNAPPY SUBSTITUTIONS: Instead of peaches, other fruits may be substituted. Some fruit, such as apples, may take longer to cook so precook fruit as necessary.