RBLs and Freemail Forwards

Hello,

on our private mail server we now have quite some forwards from freemail providers like yahoo, gmx and such. This wasn't a big problem previously but there is quite some spam arriving now over those forwards that isn't tagged as such (mainly I think because RBLs can't strike on those).

Is there away to modify the trust path such that I can actually trust the Received header added by the freemailer MTA (so that RBLs can match the Received line which is before the freemailer MTAs) ? I wouldn't really add all those to trusted hosts (and for yahoo, there are tons of mtas it seems).

Thanks in advance,

Chris