SMTP Relay

So I have a Zimbra Network Edition trial that I've been testing out, but have been having some problems with some of our users sending mail to local domains and getting spam filtered.

What's happening is that when some of our users send mail to other users in our domain, they're connecting using SSMTP-AUTH and sending mail to a domain of ours, but they're getting filtered based on their source address (which are often in dynamic ISP pools and DNSBL'ed aplenty).
Any ideas on how to get postfix not to include their source address before passing it onto delivery?

There should be an internal_networks & trusted_networks section in /opt/zimbra/conf/spamassassin/local.conf
-DNS blacklist checks will never query for hosts on these networks.

It goes in like:
internal_networks 192.168/16 127/8
trusted_networks 192.168/16 127/8

A specific IP:
trusted_networks 192.168.1.7

I believe you can also do it like:
trusted_networks 192.168.1.

If you don't want to include a particular address:
!192.168.1.54

-also be sure your zimbraMtaMyNetworks is set correctly (though you would be seeing relay access denied outright then)