Abstract: A mail slot door immobilizer for immobilizing a mail slot door within a main door, including an interior portion located on an interior side of the mail slot door and overlapping the mail slot door and the main door, and an exterior portion located on an exterior side of the mail slot door and overlapping the mail slot door and the main door. A fastener removably fastens the interior portion to the exterior portion to clamp the mail slot door and the main door between the interior and exterior portions and to prevent movement of the mail slot door with respect to the main door.