The plugin generates a PHP error:

PHP Parse error:  syntax error, unexpected '[' in /public_html/wp-content/plugins/wp-mail-smtp/class-wpms-am-notification.php on line 498,

changed said call from:

$notifications = $this->get_plugin_notifications( -1, [ 'post_status' => 'all', 'meta_key' => 'notification_id', 'meta_value' => $id ] );

to

$notifications = $this->get_plugin_notifications( -1, array( 'post_status' => 'all', 'meta_key' => 'notification_id', 'meta_value' => $id ) );

Seems to fix it