What is the best way to restrict postfix senders in Zimbra 8 so that users cannot send using a FROM address other than their own?

I've tried modifying the instruction here:
RestrictPostfixSenders - Zimbra :: Wiki

By using zmlocalconfig -e postfix_<variable_name>=<value>

To set the values (since main.cf) doesn't exist in Zimbra 8. When I look at the posfix configuration, it appears correct, but testing has proven that it is not blocking faked FROM addresses.

Any help is appreciated.