Go to the Files tab on the Mercury Core Module Configuration dialog and clear the field for Delivery confirmation template. You should also notice that there is a field for Delivery failure template. Mercury uses this template to send a notification of failure if the user doesn't exist. If you clear the confirmation field but not the failure field, a spammer could reasonably determine that if it doesn't receive a delivery failure message then the user is valid. Clearing the field would prevent that, however any person that attempts to send a legitimate email to one of your users, but accidentally misspells the email address, would not receive a failure notice and would falsely believe that their email was delivered.