I have followed the article RestrictPostfixSenders - Zimbra :: Wiki and verified the settings several times and it doesn't appear to restrict the senders to local domain users. I want to make sure that any email that is relayed through the server, even from allowed IP addresses, are restricted to MAIL FROM one of our domains. After following the wiki article I can still send out from any email account:
[zimbra@mail ~]$ telnet localhost 25
Connected to localhost.localdomain (127.0.0.1).
Escape character is '^]'.
220 mail.domain.com ESMTP Postfix
MAIL FROM: email@example.com
250 2.1.0 Ok
RCPT TO: firstname.lastname@example.org
250 2.1.5 Ok
354 End data with <CR><LF>.<CR><LF>
250 2.0.0 Ok: queued as 1E2354AB8001
221 2.0.0 Bye
Connection closed by foreign host.
Release 6.0.7_GA_2473.RHEL5_64_20100616215237 CentOS5_64 FOSS edition.