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. 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:
The device uses a photoresistor to detect the light levels in the mailbox. When the light is above a certain threshold, the Arduino knows the mailbox is open. There is a red button on the front of the device that you press to arm it. After you press the button, the next time the mailbox is opened, presumably by the mailman, you will get a text. It will not send you a text again until you have armed it again. This ensures that you don't get a text every time you open the mailbox. Every time you pick up your mail, arm the mailbox phone alert and enjoy instant mail notification.
Preliminary advice: Inform your mailman of your contraption so that he does not think you have a planted a bomb in your mailbox and use a non-metal mailbox for good cellular connection.
We'd reiterate that last bit too -- might want to let your postman know there's a sensor in the mailbox so they don't freak out and you find a bomb squad about to detonate your mailbox.
Beyond that, even if you're not obsessive about finding out when your mail arrives, a tool like this could be a good way to find out if someone's snooping in your mailbox that's not the mailman. Combined with a small camera, you may even be able to catch mail thieves in the act. The possibilities only go up from here. Hit the link below to see how the whole project was made and what you'll need to make one yourself.
Mailbox Phone Alert [Instructables]