Usually the mailboxes in the US are at the curb of the road. In order to find out if the mail-carrier was already there and if he/she did put anything into the box means: Go there and check!
The proposed setup does all that.
Principle: The LED lights up every second time the flap is closed. So, when the mail-man puts mail into the box, the light goes ON. When the user removes the mail and closes the flap - the light goes OFF. And so on. The only trick is to remember when the user puts mail into the box to be taken by the mail-man. In this case the user has to flip the flap a second time to turn OFF the light. Thus having the setup being "armed" for the mail-carrier.
ow to check the content of the mailbox from the distance. Red LED-indicator ON -> there is mail waiting.
This effect is achieved by adding a magnet to the inside of the flap (glued in) and a corresponding reed-switch at the opening. The electronic will drive the LED at the back.