[BUGFIX] use -configuration instead -settings (#800)

Co-authored-by: Dijar Bytyci <d.bytyci@frappant.ch>

Classes/Domain/Service/Mail/SendMailService.php

@@ -333,7 +333,7 @@ protected function addSenderHeader(MailMessage $message): MailMessage
         if ($this->type === 'sender') {
             $senderHeaderConfig= $this->configuration['sender.']['senderHeader.']??[];
         } elseif ($this->type === 'receiver') {
-            $senderHeaderConfig = $this->settings['receiver.']['senderHeader.']??[];
+            $senderHeaderConfig = $this->configuration['receiver.']['senderHeader.']??[];
         }
 
         $email = ObjectUtility::getContentObject()->cObjGetSingle(