Blocking redirect/forwards to external domains?

[gdavid]

Hi guys. Many customers are using forwarding in our ISPConfig server to receive their email on mailboxes not belonging to our mailserver.
The problem is that if spam arrives in our mailserver, this spam is forwarded to onother mailserver and, after a while, our mailserver is classified as spammy by various blacklists.
This happends because the spammy forwarded email seems, in fact, originated by our mailserver.
How can we configure postfix (or ISPConfig) to drop emails forwarded to external domains?
Does anyone already messed up with this problem?
Thanks
g