Hi Benoit
In some cases it seems nearly impossible to me to prevent backscatter if you got email forwardings.
How do other ISP solve that problem? Just tell you customers you won't offer email forwarding any more?
Forwarding to a sigle address would ideally be done with your mailserver acting as a proxy, i.e. opening a smtp connection to the final destination server and forwarding the status codes to the sender. However you'll most certainly get in trouble if the recipient mailserver is using SPF. And as you said, forwarding to multiple addresses is a bit tricky...
Maybe you need to trade backscatter for mailbox storage: forward all mails with the envelope from of a local mailbox of the user and have that user check his mailbox at your place from time to time.
I'm afraid I don't see a solution without having either backscatters or using local storage for the "bounces".
Cheers
Jean-Pierre