There was a project in a magazine years ago(maybe Popular Electronics). The part in the mailbox was a circuit that used just a switch, capacitor and battery.Since the capacitor blocks DC current flow the circuit only produces a pulse and the battery will last its shelf life. The part in the house was a simple SCR circuit that would turn on and latch whenever it received a pulse from the mailbox any time the door was opened or closed. I built one for my grandfather who was a gadget nut. It worked well with the exception that it got a few false triggers We figured out lightning strikes was one source and CB radios may have been another.
Jimmie