Intumescent Letterbox Assemblies 30 min\'s Fire Protection. 305mm BS EN 1935. Smokeproof. Comes with a 10 year mechanical guarantee and is Fire door rated. A stylish ABS framed, adjustable telescopic sleeved letterbox, with metal flaps and fitted with a high performance intumescent liner. In the event of fire the intumescent liner expands rapidly and seals off the letterbox opening, preventing the spread of fire and smoke whilst maintaining the integrity of the fire door. An all-in-one letter plate solution for your front door. This letterbox assembly has an identical appearance from inside or ...