Permission "administer smtp module" does not exist

Currently in the D8 version of the module, SMTP defines it's permission using hook_permission().

function smtp_permission() {
  return array(
    'administer smtp module' => array(
      'title' => t('Administer SMTP Authentication Support module'),
      'description' => t('Perform administration tasks for SMTP Authentication Support module.')
    ),
  );
}

However, permissions should be defined in the smtp.permissions.yml file.

More information about this change can be found here.