Fix PHP notice when property doesn't exist (#2863)

class/deliver/Deliver.class.php

@@ -741,7 +741,7 @@ class Deliver {
             /* RFC 2298 */
             $header[] = 'Disposition-Notification-To: '.$dnt. $rn;
         }
-        if ($rfc822_header->dsn) {
+        if (isset($rfc822_header->dsn) && $rfc822_header->dsn) {
             $dsn = $rfc822_header->getAddr_s('dsn');
             $header[] = 'Return-Receipt-To: '.$dsn. $rn;
         }