I need to allow users to forward mail off my server. How can I protect my server's reputation?
While you will not be able to protect your reputation completely with email forwarding enabled, there is an action you can take to mitigate the danger.
Enable SpamAssassin for forwarded mail.
We have two ways to use this feature. You can accept the default scoring method by using the option "Do not forward mail to external recipients if it matches the Apache SpamAssassin™ internal spam_score setting." If you're familiar with SpamAssassin, you can set your preferred score with the option "Do not forward mail to external recipients based on the defined Apache SpamAssassin™ score."