Send site emails to multiple administrators
This is much similar to Pontus's answer but in a different approach. and I have tested this and it worked on a Ubuntu server email program (not through an external SMTP server but it should work in a decent program). Pontus's solution will work as it seems. But recipient will see both email addresses. A good approach would be to set the BCC address (Blind Carbon Copy) instead of a new CC.

If you are not willing to make a module, I'd suggest http://drupal.org/project/maillog .

function MYMODULE_mail_alter(&amp;$message) {
 if (-- enter your condition here -- ) {
   $message['headers']['BCC'] = $message['headers']['Bcc'] = 'hidden@example.com';
 }
 }


Note that now you have opened a serious security risk that hidden@example.com can now see any email, including password reset email address for the super user. So don't forget  to add some condition using $message['id] that contains the message's ID. 

You can make the module fancy and really usable by adding some sort of system_settings_form()'d form and a few variable_get().

Also, if the purpose of this is to send multiple emails on new security releases for modules, don't forget that you can add multiple email addresses in settings out-of-the-box.