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?