Change from email id when admin approved user account
I don't have enough reputation to comment, so I am expanding user1081275's answer - 

The message id for the confirmation message you want to alter is 'status_activated'. 

You can intercept outgoing mail with hook_mail_alter() in a custom module as described at https://api.drupal.org/api/drupal/modules!system!system.api.php/function/hook_mail_alter/7

function mycustommodule_mail_alter(&amp;$message) {
  if ($message['id'] == 'status_activated') {
    $message['from'] = "mail@whatever.com";
  }
}


Per user.module, the ids for other messages defined by the User module in _user_mail_notify() are:

 *   - 'register_admin_created': Welcome message for user created by the admin.
 *   - 'register_no_approval_required': Welcome message when user
 *     self-registers.
 *   - 'register_pending_approval': Welcome message, user pending admin
 *     approval.
 *   - 'password_reset': Password recovery request.
 *   - 'status_activated': Account activated.
 *   - 'status_blocked': Account blocked.
 *   - 'cancel_confirm': Account cancellation request.
 *   - 'status_canceled': Account canceled.