Welcome
Well first things first ... Have you checked /var/log/zimbra.log for any error messages ?
With postfix not starting it will be one of two reason 1) existing MTA running on port 25 2) Your DNS is wrong. So with that is mind please post the following information
Code:
netstat -an | grep 25
cat /etc/hosts
cat /etc/resolv.conf
dig yourdomain mx
dig yourdomain any
host `hostname` <- note backticks and not single quotes