List of flags :
        u -> unread                 f -> flagged
        a -> has attachment         s -> sent by me
        r -> replied                w -> forwarded
        d -> draft                  x -> deleted
        n -> notification sent

        by default a message priority is "normal" otherwise:
        ! -> priority high          ? -> priority low

def update_messages_flag(self, ids, flag):
        """
        List of flags :
        u -> unread                 f -> flagged
        a -> has attachment         s -> sent by me
        r -> replied                w -> forwarded
        d -> draft                  x -> deleted
        n -> notification sent

        by default a message priority is "normal" otherwise:
        ! -> priority high          ? -> priority low
        """
        str_ids = self._return_comma_list(ids)
        params = {'action': {
            'id': str_ids,
            'op': 'update',
            'f': flag
        }}

        self.request('MsgAction', params)