Add functionality of href for subscribe and opt-out in addition to incoming mail for CiviMail.
Mails that are send using CiviMail should contain href for unsubscribe and optout.
so when user clicks on those links he comes to civicrm.
create two file /extern/unsubscribe.php and /extern/optout.php
once he comes to civicrm display unsubscribe or opt out message.