I wanted to put a motion sensor in my mail box so when motion is detected I get a "you've got mail" notification on the smart speakers. It would be such a novelty
Unfortunately, after researching motion sensors (and let's not forget the Australian summer often being an ambient 40C outdoors - and batteries not liking that) - it appears that they detect organic motion, as opposed to inanimate objects moving, particularly in a letter box.
It is still something I want to do one day, and my wifi just reaches my letter box. :P