Fatal error: Uncaught Error: Interface "Drupal\Core\Mail\MailInterface" not found in /home/www/drupal8/test/modules/contrib/htmlmail/src/Plugin/Mail/HTMLMailSystem.php:30
Stack trace:
#0 {main}
thrown in /home/www/drupal8/test/modules/contrib/htmlmail/src/Plugin/Mail/HTMLMailSystem.php on line 30