If you've ever checked the mail repeatedly only to find it empty, or
your mailbox is a good hoof from your house, this project is for you.
It's a simple photo-sensitive alarm that will fire off an SMS message
to your phone when the mailbox is opened and the sensor is triggered.
The video above shows you how it works, and while it definitely
requires some specific electronics to make it work, it's nothing you
can't do easily with a little effort. Here's how Instructables user
nikoala3 describes it:
How well would your router work for detecting the mailman? I was
thinking something like having a sensor on the delivery vehicle so
when the mailman gets within range of your house, it would detect it.