procmail as multidomain spam filter

Is it possible to set up procmail as general purpose, multi-account, 
multi-domain spam filter / relayer, in the manner of 
www.MailShield.com.  ie, I'd like to point multi MX records at a dedicated 
procmail machine, have procmail filter all msgs of all domains for spam, 
and then transparently forward passed msgs to the various destinations SMTP 
servers.
If not, could somebody recommend an opensource approach to mailshield type 
of machine?
Thanks,
Len