Pending approval mail not sent
Check these variables:

user_mail_register_admin_created_notify
user_mail_register_no_approval_required_notify
user_mail_register_pending_approval_notify
user_mail_status_blocked_notify
user_mail_cancel_confirm_notify
user_mail_status_canceled_notify


Use drush (drush vget user_mail_register_no_approval_required_notify) or install Mail Control; This module provide only interface modifications. The underlying functionality is already there by default in Drupal 7.

You can also drop in settings.php:


$conf['user_mail_register_admin_created_notify'] = FALSE; 
$conf['user_mail_register_no_approval_required_notify'] = FALSE; 
$conf['user_mail_register_pending_approval_notify'] = FALSE; 
$conf['user_mail_status_activated_notify'] = FALSE; 
$conf['user_mail_status_blocked_notify'] = FALSE; 
$conf['user_mail_cancel_confirm_notify'] = FALSE; 
$conf['user_mail_status_canceled_notify'] = FALSE;