Follow the steps suggested in postfix wiki will be able to restrict a list of users for sending emails to limited domains.
To add on on the wiki , we can replace the ip address with the user email address. For example :
#Add the list of IP addresses which cannot send emails outside
change it to
The rest is the same , just try to restrict by user account instead of ip address.
Zimbra is using postfix as it smtp daemon , so these two guides might help us to have better understanding :