[CentOS] Advice, Please.

Hello,

I have a home setup where I am using Centos 4.4 as a sort of mail server.  
This is what I have done.

I have it configured to pop mail locally.  I have fetchmail configured to dl 
from my provider my families mail through a cron job and .fetchmailrc in 
their home directories.  They then pop the server to retrieve their mail.  
They send their mail directly through the provider.

I have been relying on spamassasin on their local boxes to learn and filter 
out the spam but find it is only doing about half of the job even after 
months of training.

I guess my question is this.  Is there a way to do further spam filtering on 
the mail server incorporating my method of doing mail or am I not just doing 
this right?

Any thoughts, please?  Thank you,

Phil