rbl not working with spamassassin

[stefano at cdh.it]

I've just added spamassassin in a long working ( and happy :) ) mail
scanner installation.
It works flawlessy only if mailscanner do the RBL check; if I activate
spamassassin rbl following the exact advice given here
(http://www.sng.ecs.soton.ac.uk/mailscanner/serve/cache/112.html)
spamassassin stops working at all.
do you have any hint?
thanks
Stefano