--- system.api.php	2009-07-27 11:44:56.000000000 -0700
+++ system.api.edit.php	2009-07-27 11:49:18.000000000 -0700
@@ -626,7 +626,7 @@ function hook_profile_alter(&$account) {
  */
 function hook_mail_alter(&$message) {
   if ($message['mail_id'] == 'my_message') {
-    $message['body'] .= "\n\n--\nMail sent out from " . variable_get('sitename', t('Drupal'));
+    $message['body'][] = "\n\n--\nMail sent out from " . variable_get('sitename', t('Drupal'));
   }
 }