fix for BIESt 3523, closes #3523

Closes #3523

Merge request studip/studip!2408

lib/models/MailQueueEntry.class.php

@@ -123,6 +123,7 @@ class MailQueueEntry extends SimpleORMap
     {
         $mail = new StudipMail($this->mail);
 
+        if ($mail->getRecipients()) {
             $success = $mail->send();
             if ($success) {
                 $this->delete();
@@ -131,7 +132,10 @@ class MailQueueEntry extends SimpleORMap
                 $this['last_try'] = time();
                 $this->store();
             }
-
+        } else {
+            $success = false;
+            $this->delete();
+        }
         return $success;
     }
 }