My mailbox is several walls away from my flat and has no power outlets nearby. I used a Zigbee motion sensor and flashed a Sonoff Dongle-E as a battery-powered router to extend the mesh.
One unexpected issue was the power bank shutting down due to low power draw, which I fixed using a small USB dummy load.
Full write-up with diagrams and setup: https://jonathas.com/how-i-made-my-physical-mailbox-send-me-...