My mail runs at odd times, and it's not fun hiking out to the mailbox and finding it empty, so I decided to make a notifier using a ESP8266 NodeMCU board.
I wrote some code for the ESP8266 that sends me an email over wifi when a sensor is tripped (I put the
code on Github if you're interested).
Next, I just need to: find something to house the project in, pick out a battery, buy a reed switch, make sure my wifi reaches to my mailbox (or find a way to extend it), and install.