The plan here is to develop a permanently supplied (no batteries, only a super cap) incoming mail system. I plan to use a "line finder" sensor to sense for new mail (Ir LED + photodiode); the photodiode would then "wake up" a microcontroller from sleep mode (a teensy LC for example), that would then send a [new mail] message to a base station.
The base (receiving) station will be inside home and will be supplied by the power outlet. A color LED will light every time new mail comes.
There are some hackaday projects with that same objective, and also a couple of U.S. patents:
An incoming mail detection system needs a sensor - I decided to start prototyping with an infrared LED and sensor: the SeeeStudio Grove "line-finder" that I bought here and a Teensy LC that I got from http://hackaday.io some weeks ago (inside a 2015HackadayPrize giveaway!).
It worked just out of the box, using a code that can be found on my GitHub (link in this project page). You can see a video of the first test below: