Summer Meals Alternative Relief and Transportation Act of 2021 or the SMART Act of 2021 This bill expands the Summer Food Service Program, which provides meals to children and teens in low-income areas during the summer months when school is not in session, to include a home-delivery option in certain states. Qualifying states must have areas in which meals are inaccessible at program sites and in which electronic benefit transfer funds are not able to be fully used (e.g., due to the distance of eligible retailers). The Department of Agriculture must borrow from similar demonstration projects in its administration of the expanded program (e.g., the Meals-To-You Demonstration Project).