Deprecated using a function callback for sending mail; should be done by...

Closes #3106043

src/Plugin/QueueWorker/MandrillQueueProcessor.php

@@ -3,6 +3,7 @@
 namespace Drupal\mandrill\Plugin\QueueWorker;
 
 use Drupal\Core\Queue\QueueWorkerBase;
+use Drupal\Core\Queue\RequeueException;
 
 /**
  * Sends queued mail messages.
@@ -30,7 +31,10 @@ class MandrillQueueProcessor extends QueueWorkerBase {
     /* @var $mandrill \Drupal\mandrill\MandrillService */
     $mandrill = \Drupal::service('mandrill.service');
 
-    $mandrill->send($data['message']);
+    $result = $mandrill->send($data['message']);
+    if(!$result){
+      throw new RequeueException('Mandrill send request had an exception!');
+    }
   }
 
 }