Ok problem solved.
I bought a Fibaro door module (magnetic sensor), powered by a 3.6V battery all included.
Right now my problem is Zwave radio range, not reaching the mailbox.
I'm gonna try to solve that problem when installing the fibaro shutter modules for the blinds, that supposedly repeats the Zwave signal.
One of the shutter modules will be almost directly above the mailbox so I hope it has enough range and works!
Cheers!