Is gform_paypal_fulfillment fired when using Sandbox Texting Environment ?

I can't manage to trigger a function using gform_paypal_fulfillment.
Is it fired when using the Testing Mode of Paypal ?

Here is the basic function I use :

add_action("gform_paypal_fulfillment", "paypal_payment_handler", 10, 4);
function paypal_payment_handler($entry, $config, $transaction_id, $amount){
$my_email_addr = 'emailadress@domain.com';
$message = "ID=".$entry['id'].", post_id=".$entry['post_id'].", Amount=".$amount;
mail($my_email_addr, 'payment', $message);
}