The follwing are the simple steps to blocking users in zimbra postfix.
Below mentioned are the steps to "REJECT" an external email address from sending mail to the users of the Zimbra Domain.
1. Edit /opt/zimbra/conf/zmmta.cf
Add this line below (smtpd_recipient_restrictions)
POSTCONF smtpd_sender_restrictions FILE postfix_sender_restrictions
2. Create file /opt/zimbra/conf/postfix_sender_restrictions with the below line:
3. Create file /opt/zimbra/postfix/conf/reject with the list of email address to be rejected in the below format:
4. postmap /opt/zimbra/postfix/conf/reject
5. zmmtactl stop ; zmmtactl start
Please note :- This change will not survive an upgrade and you will have to redo these after the upgrade.
Courtesy :- http://wiki.zimbra.com/index.php?title=Domain_level_blocking_of_users