I tried posting this here, but it did go through. Anyhow, I created a posting on our blog with all the details
here:.
Quick summary:
1) Download and install Pop-before-smtp
2) Change /opt/zimbra/conf/log4j.properties.in
3) add a line at the beginning of opt/zimbra/conf/postfix_recipient_restrictions.cf
4) Restart zimbra and start the pop-before-smtp daemon.