code: def validate_maildirs(ctx, param, value):
      
      for path in value:
          for subdir in MD_SUBDIRS:
              if not os.path.isdir(os.path.join(path, subdir)):
                  raise click.BadParameter(
                      '{} is not a maildir (missing {!r} sub-directory).'.format(
                          path, subdir))
      return value
docs: Check that folders are maildirs.