Failure notification not working ?

Hello !

I am testing the service (using smtp method) and I can send emails correctly.
But if I send an email to an address that doesn’t exist (failure is garanted : It tested it with gmail), I don’t get failure notification on my admin email address (although this is set so in failure notification settings).

So : do you know what kind of failure this notification system handles ?
How can I have notification for every mail that does not arrive to its customer ?

Thank you for your help !

Best regards