--- common.inc.old	2007-03-23 20:49:11.000000000 +0000
+++ common.inc.new	2007-03-28 12:28:33.000000000 +0100
@@ -1924,6 +1924,7 @@
     'Content-Transfer-Encoding' => '8Bit',
     'X-Mailer' => 'Drupal'
   );
+  $defaults['Return-Path'] = variable_get('site_mail', ini_get('sendmail_from'));
   if (isset($from)) {
     $defaults['From'] = $defaults['Reply-To'] = $defaults['Return-Path'] = $defaults['Errors-To'] = $from;
   }
@@ -1967,7 +1968,8 @@
       $to,
       mime_header_encode($subject),
       str_replace("\r", '', $body),
-      join("\n", $mimeheaders)
+      join("\n", $mimeheaders),
+      '-f'.$defaults['Return-Path']
     );
   }
 }