Setting Send and Recieve Restrictions

[csyperski]

I work for a High School District which is considering using Zimbra for 4000 student email accounts, but I am having a problem setting some postfix configs. I have to setup zimbra to restict students so they can send and recieve from the domain in which they belong plus a few white listed domains. I am attempting to modify the
smtpd_recipient_restrictions
to include "check_sender_access hash:/opt/zimbra/postfix/conf/restricted_senders"
but it seems that everything I restart zimbra this setting is reset to
"smtpd_recipient_restrictions = reject_non_fqdn_recipient, permit_sasl_authenticated, permit_mynetworks, reject_invalid_hostname, reject_non_fqdn_sender, reject_unauth_destination, permit"

Am I not going to be able to modify this in postfix? Is there a easier way to block sending and recieving to all domains with the exception of a few whitelisted sites. Thanks in advance for any help that you can offer.

[marcmac]

grr. cross-posting