I have been having a lot of MTA issues since upgrading to Zimbra OSS 6.x from 5.x. I have made a few posts but nothing really worked. So I thought I would post what I discovered.
I found that if I renamed /etc/postfix to /etc/postifix.orig and symlink the /opt/zimbra/postfix/conf to /etc/postfix everything went happy again. It's as if the postfix binaries were compiled ( I really don't know ) with a wrong prefix for the config files.
I'm running Zimbra 6.0.x on Mac OS X Sever 10.5.8.