Can a fire door have a letterbox?

Yes, as long as it's a fire-rated one that suits the door. A plain slot cut through the leaf takes timber out of the very place that has to hold, and it leaves a gap smoke can walk through. A rated letterbox has an intumescent lining that swells in the heat and closes that gap, and usually a flap behind it as well.

On an inspection we check whether the letterbox has a lining and a flap, and whether it looks like part of the door or like something a resident cut in later. A ragged cut, a gap round the frame of the box or a worn-out brush all go on the report.

If one is wrong, what happens next depends on the door. Some can take a rated unit. On others it's cleaner to block the hole, fit a post box on the wall outside the flat and deal with the door itself separately. The report tells you which, door by door.