Domain level blocking of users
A method to restrict specific emails from sending mails to the domain exist via blacklist :http://wiki.zimbra.com/index.php?title=Improving_Anti-spam_system#Implementing_Whitelist.2FBlacklist
Follow the below method if you wish to achieve the same via postfix.
1. Edit /opt/zimbra/conf/zmmta.cf
Add this line below (smtpd_recipient_restrictions)
POSTCONF smtpd_sender_restrictions FILE postfix_sender_restrictions
2. vi /opt/zimbra/conf/postfix_sender_restrictions
hash:/opt/zimbra/postfix/conf/reject
3. Create file /opt/zimbra/postfix/conf/reject with the list of email address to be rejected in the below format:
user@domain.com REJECT
4. postmap /opt/zimbra/postfix/conf/reject
5. zmmtactl stop ; zmmtactl start