def _set_metadata(self, value):
        '''
        A special set function to ensure
        we're setting with a dictionary
        '''
        if value is None:
            setattr(self, '_PMMail__metadata', {})
        elif isinstance(value, dict):
            for k, v in value.items():
                if (not isinstance(k, str) and not isinstance(k, int)) \
                    or (not isinstance(v, str) and not isinstance(v, int)):
                    raise TypeError('Metadata keys and values can only be strings or integers')
            setattr(self, '_PMMail__metadata', value)
        else:
            raise TypeError('Metadata must be a dictionary of key-value pairs')