I am very much interested in a solution to this problem, too. SpamAssassin classifies e-mails as spam just because the X-Originating-IP, which is usually a dynamic IP of a user, is listed in some DNS BLs, which is something a regular user usually does not know nor care about, and there is nor reverse DNS for this IP, of course.
You can disable the X-Originating-IP header by going into Webmin -> Webmin -> Usermin Configuration -> Usermin Modules -> Read Mail, and set “Include browser IP in X-Originating-IP header” to “No”.