how to pass spamassassin on smtp authenticated users


I’m facing the following problem lately (after tunning spamassassin). Some of my users are connecting to the mail server through mobile phones and the leased IPs from the GSM operators are blacklisted in spamhaus and spamcop. So, they are using the smtp server but their messages are marked as spam and not delivered, since the rbl checks are positive.

Is there a way to “trust” smtp authenticated users ?