The hardware itself is the popular Arduino board, the tinkerer’s dream device. I’m actually using a Boarduino, but any variant should work (subject to a small but important detail, see below). This might be particularly interesting with a Bluetooth Arduino..

The Arduino talks with your computer over a serial connection, which runs over the normal USB cable you use to communicate with your Arduino.

This project requires two bits of software: one running on the Arduino to receive serial data and toggle the output accordingly, and one running on the computer to check for new emails and send data out over the serial port. To re-iterate, these code listings should only be used as a guide. They work for me, but I don’t guarantee they’ll work for you, and I accept no responsibility for breaking things. If you don’t understand what this code is doing, you probably shouldn’t be running it.

The Arduino software is fairly trivial. It just loops round, watching for bytes coming in over the serial port, and sets a variable called “mail” to be true if the data is an ‘M‘, and false if it’s an ‘N‘ (of course these characters are arbitrary). It then updates the value of the output pin to be equal to this mail variable. You can hack this and expand it or modify it however you like - perhaps you could make an LED blink when you have new email instead of just turn on?