To start receiving IPN messages you have to login into your account, go to Merchant Services >> IPN Settings, select "Turn ON IPN", enter notification URL and select "Receive IPN messages".
To temporarily stop receiving IPN messages, select "Do not receive IPN messages". We will continue to generate and store IPN messages until you select "Receive IPN messages" again ( or turn off IPN ). To completely stop IPN messages you must click on "Turn OFF IPN".
To validate that the incomming IPN messages are valid you also need to generate a Shared Secret key on the IPN Settings page.