Preheat the oven to 350F. Spray a 8.5 x 4.5 or 9 x 5 pan with baking spray. Or, butter the pan, sprinkle with flour, and tap out the excess flour over the sink.
In a medium bowl, whisk together the 1 ⅔ cups of flour, baking powder, salt, and poppyseeds. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter, sugar, and grapefruit zest on medium-high speed until light and fluffy, about 3-5 minutes.
Add the vanilla. Add the eggs one at a time, scraping the bowl after each one has been added.
With the mixer on low speed, add the flour mixture to the batter in the three additions, alternating with the milk in two additions, beginning and ending with the flour. Mix until just combined.
Pour the batter into the pan and bake 45-55 minutes or until a toothpick stuck in the loaf comes out with few moist crumbs, but no wet batter.
Cool the loaf in the pan for 15 minutes on a cooling rack. Remove the loaf from the pan and place on the cooling rack to continue cooling.