Eric Tsai made a similar project using an Arduino compatible MCU, he's using the RFM69HW wireless module for data transmission, just like the Moteino, it might be a better idea than the NRF24L01, but more expensive...

As far as the sensor for detecting new mail delivery is concerned my first idea was a light sensor to detect the mailbox opening (while the module is inside the mailbox), but I figured since power consumption must be minimised the sensor should be a passive one. So I'll just use a button based "sensor" that'll trigger an interrupt and wake up the MCU, this way the "sensor" won't draw any power whatsoever while waiting.