Thanks for the reply. Now I know why it's not working.
You are right about "solve at source" but we cannot force our customers to mod their mail server or hardest to modify the mail server of a service provider.
I just took the patch of SpamC fine tune. I removed the "reject invalid reverse DNS" flag and raised the score of RDNS_NONE and RDNS_DYNAMIC.
This way i far better because emails with problem in rDNS are placed in quarantine and not rejected.
Now I'm working on spamassassin's rules score to setup the best filter.
Thanks Reivilob!