[CentOS] Restrict User on Sending Mail

I search from google on how to restrict user from
sending mail and I did trial and errors for three days
already but to no success. What I want to do is to
restrict some user in sending mail while some
unrestricted user will send mail to internet and
intranet. I am able to restrict outside address (i.e.
gmail, yahoo) to not send mail on restricted users but
the vice versa I can not do. Is there anybody here
that have experience in doing so. Please help. By the
way, I am using Postfix on top of CentOS 4.5. Below
are some of the entries in main.cf:

smtpd_recipient_restrictions =
        check_recipient_access
hash:/etc/postfix/access,
        reject_invalid_hostname,
        reject_unknown_recipient_domain,
        reject_unauth_pipelining,
        permit_mynetworks,
        permit_sasl_authenticated,
        reject_unauth_destination

smtpd_client_restrictions = check_client_access
hash:/etc/postfix/restricted_sender
#smtpd_sender_restrictions = check_sender_access
hash:/etc/postfix/restricted_sender

/etc/postfix/restricted_sender contains:
testuser1 at mydomain.com REJECT

I already try to use smtpd_client_restrictions and
smtpd_sender_restrictions with no success, is there
any other way to do it. Thanks.

junji
aisalen.wordpress.com
Linux Registered User #253162

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