From time to time serious house fires are caused by flamable substances being pushed through letter boxes. Homes in many other countries do not have these apertures in their front doors, but instead have a separate metal box or cassette of boxes if flats.
If provision of a letter box is a requirement, now that the post office counters have a plastic size template limiting the size for first class minimum postage letter to a narrow slot of 165 mmm x 5 mm, perhaps we can reduce our aperture to that size and still meet the regulation.
Regrettably even this would not stop junk mail coming through it.
Roger R