static void mail_listFolders(Folder folder) {
  mail_listFolders(folder, "  ");
}

static void mail_listFolders(Folder folder, S prefix) ctex {
  Folder[] folderList = folder.list();
  for (int i = 0; i < folderList.length; i++) {
    print(prefix + folderList[i].getFullName());
    mail_listFolders(folderList[i], prefix + "  ");
  }
}