Index: modules/system/system.api.php
===================================================================
RCS file: /cvs/drupal/drupal/modules/system/system.api.php,v
retrieving revision 1.58
diff -u -p -r1.58 system.api.php
--- modules/system/system.api.php	2 Aug 2009 11:25:18 -0000	1.58
+++ modules/system/system.api.php	5 Aug 2009 22:53:00 -0000
@@ -618,8 +618,8 @@ function hook_profile_alter(&$account) {
  *     MIME-Version, Content-Type, etc.
  */
 function hook_mail_alter(&$message) {
-  if ($message['mail_id'] == 'my_message') {
-    $message['body'] .= "\n\n--\nMail sent out from " . variable_get('sitename', t('Drupal'));
+  if ($message['id'] == 'modulename_messagekey') {
+    $message['body'][] = "--\nMail sent out from " . variable_get('sitename', t('Drupal'));
   }
 }