If there were a string in the mail slot in the house where I grew up it would not end well, unfortunately. Even the mail slot itself is suspect since these have been used by burglars to open unlocked doors (metal wire through the slot, easy), by vandals to throw in fireworks (one house burned down that way), by creeps to pee through it (another house in the same area), by other vandals to put a garden hose in to flood the hallway just for kicks, etc.

This was in Amstelveen, to the south of Amsterdam. Where do you live?