def by_mail_folder_id1(self,mail_folder_id1: str) -> MailFolderItemRequestBuilder:
        """
        Provides operations to manage the childFolders property of the microsoft.graph.mailFolder entity.
        param mail_folder_id1: The unique identifier of mailFolder
        Returns: MailFolderItemRequestBuilder
        """
        if mail_folder_id1 is None:
            raise TypeError("mail_folder_id1 cannot be null.")
        from .item.mail_folder_item_request_builder import MailFolderItemRequestBuilder

        url_tpl_params = get_path_parameters(self.path_parameters)
        url_tpl_params["mailFolder%2Did1"] = mail_folder_id1
        return MailFolderItemRequestBuilder(self.request_adapter, url_tpl_params)

        Provides operations to manage the childFolders property of the microsoft.graph.mailFolder entity.
        param mail_folder_id1: The unique identifier of mailFolder
        Returns: MailFolderItemRequestBuilder
        
msgraph/generated/users/item/mail_folders/item/child_folders/child_folders_request_builder.py