Maybe I’m wrong. Postfix has features to recognize and discard forged sender adresses. Are they used on nethserver ?
From the official doc : http://www.postfix.org/BACKSCATTER_README.html#random
By the way : It just came to my mind that that issue (spam mails bounced) already plagued me in the past : I use a smarthost that decides from time to time that I’m a spammer because I send too many non delivery notifications for spammers ! Read my post I wrote at that time on this board : Postfix sending non-deliveries notifications because of spam?
That’s a real issue, we should handle it.