The configuration schema uses the wrong type for the root object

The configuration schema is using the wrong root object type.

graph_mail.settings:
  type: config_entity
  label: 'Graph Mail settings'

As made clear in the Drupal 9 configuration schema cheat sheet file, for a single configuration file, the type to use is config_object, not config_entity, which is instead used when the same schema is used by multiple files.