There are a couple of customizations that can only be done by editting the /opt/zimbra/postfix/conf/main.cf file. The problem is, if you do any upgrades, you loose your changes. After being burned by this, I added this script to my servers

In my case, I needed all inbound email copied to an archive mailbox. So, first I created a file called /root/bin/UpdateMain.cf

#!/bin/bash
target=/opt/zimbra/postfix/conf/main.cf
f=`grep -i -c "always_bcc=archivex@mydomain.com" $target`
if [ $f = 0 ]; then
    echo "always_bcc=archivex@mydomain.com" >> $target
    su zimbra -c "~/bin/postfix reload"
fi

Make sure you

chmod 777 /root/bin/UpdateMain.cf

Then:

crontab -e

and add the line

10 * * * * /root/bin/UpdateMain.cf

(you can change the interval if you wish)

and now – you never loose your changes to main.cf!