Email Bounce event

Hi , I have callback listener setup and configured to trigger when there is "PACKAGE COMPLETE" , "PACKAGE DECLINE" and "EMAIL BOUNCE" events. While it works fine with the "PACKAGE COMPLETE" and "PACKAGE DECLINE" events, I am not able to get a response for "EMAIL BOUNCE". I know this is an inavalid email and so this event should be triggered when the signer email is none existent. Any idea why this would be the case. Example signer email: [email protected] Thanks, Sudhangi