Index: wp-includes/class-phpmailer.php
===================================================================
--- wp-includes/class-phpmailer.php	(revision 19679)
+++ wp-includes/class-phpmailer.php	(working copy)
@@ -1141,9 +1141,7 @@
 
     $result .= $this->HeaderLine('Date', self::RFCDate());
     if($this->Sender == '') {
-      $result .= $this->HeaderLine('Return-Path', trim($this->From));
-    } else {
-      $result .= $this->HeaderLine('Return-Path', trim($this->Sender));
+      $this->Sender = trim($this->From);
     }
 
     // To be created automatically by mail()