Some of my mail users are forwarding their mails to gmail for example. In some cases my Amavis lets some spam messages slip through. These mails are then bounced by gmail as they are spam. My postfix then tries to send non-delivery notifications to the (forged) from addresses, known as backscatter.
How can I stop postfix from sending non-delivery notification for mails that have bounced while forwarding them? Note I don't want to completely disable non-delivery notifications, only when forwarding mail.