Global filtering for a site

I would like to implement global filtering of incoming mail for a site
running sendmail - this is to block a few outsiders from spamming the
site - let's avoid the discussion of if social engineering methods are
more effective or appropriate.  I know that I can filter locally
delivered through procmail on a sitewide basis - but what do I do
about forwarded mail to outside hosts - which would then be delivered
using the smtp mailer instead ?

Any ideas ?  

Vince Taluskie