Fatal error: Uncaught Error: Class "Drupal\Core\Mail\Plugin\Mail\PhpMail" not found in /home/www/drupal8/test/modules/webform/src/Plugin/Mail/WebformPhpMail.php:17
Stack trace:
#0 {main}
thrown in /home/www/drupal8/test/modules/webform/src/Plugin/Mail/WebformPhpMail.php on line 17