Function delete_mail_folder 

pub async fn delete_mail_folder(
    http: &Client,
    base_url: &str,
    access_token: &str,
    folder_id: &str,
) -> Result<(), ClientError>

DELETE /me/mailFolders/{id}: delete a folder. Outlook moves the folder (and any contents) to Deleted Items, so this is recoverable. Works for top-level and child folders alike.