def sender(self, value):
        """ sender is a property to force to be always a Recipient class """
        if isinstance(value, Recipient):
            if value._parent is None:
                value._parent = self
                value._field = 'from'
            self.__sender = value
        elif isinstance(value, str):
            self.__sender.address = value
            self.__sender.name = ''
        else:
            raise ValueError(
                'sender must be an address string or a Recipient object')
        self._track_changes.add('from')