How do I make a notification of new mail in a mailbox with ESP and Supla module?

Hello
I would like to use the esp module and using wifi to make a notification that there is mail in the mailbox to be received in some simple and effective way to make a notification of new mail and connect this to the suple. I live in a block of flats the mailbox is quite small and there is no handle or arm to do this on a microstick. I was thinking more of a sensor which will detect the letter, switch on the esp and I will see in the supple application that something is in the box, power supply I was thinking of doing on rechargeable batteries e.g. 2pcs one 3.7v 3400mA. There are many different sensors out there, I just don't know which would be the most optimal for this project.

Hello !
The mailbox is plastic ?
Wi Fi through metal will not pass.
Buy the module you have shown, mount the light sensitive elements perpendicular to the board and test, you have to start somewhere.

This is what I feared, that the metal would jam the signal. I'll keep trying, thanks for your help. Do I need to buy any more light sensitive components or is the sensor alone sufficient for detection?

There are photosensitive elements on this board. If you haven't already bought this board then look for a similar board with photosensitive elements mounted perpendicular to the board.

Ok thanks for the info I will be looking for one. I am planning to order a few sensors of different operating principle and test this hopefully it will achieve its purpose.

Is it possible to get a range with such a transmitter and receiver, because with wifi I'm afraid the signal will interfere. I read somewhere on the forum that someone achieved such an effect even in a block of flats through reinforced concrete walls by attaching wires as antennas to a box I have two floors below.