Ignore locally sent email - how?

I would like for dcc to not consider, transmit, etc. locally sent  
email. By locally, I mean from cron, or other program on the Linux  
host itself. The mail system is postfix, and, we are using dcc via  
Spamassassin and procmail at delivery time.

So, when mail comes from say a cron job, the ONLY received header is  
this:

Received: 	by host113.changedname.com (Postfix, from userid 0) id  
4A8E114B8104; Tue, 15 Sep 2009 03:53:19 -0500 (CDT)

So, There is no FROM generated by Postfix. So, I am not sure how to  
whitelist these emails so when delivery occurs to local mail users,  
they do not get analyzed by dcc.