Reading at http://www.postfix.org/postconf.5.html#smtpd_relay_restrictions, we see that, by default, network mail relay access to postfix is restricted to "local" or "authenticated" users:
smtpd_relay_restrictions (default: permit_mynetworks, permit_sasl_authenticated, defer_unauth_destination)
Access restrictions for mail relay control that the Postfix SMTP server applies in the context of the RCPT TO command, before smtpd_recipient_restrictions.
At the same time, though, any user able to run applications on the mail server can bypass these network smtpd_relay_restrictions, even the SASL authentication requirement, by simply using the local postfix sendmail command to send mail. The postfix sendmail command might be run directly, from a shell, or indirectly, for example, using the Gnome Evolution "personal information manager".
Rather than restricting access to the postfix sendmail command itself, by changing the sendmail user or group permissions and per-user group memberships, how can postfix be directly configured to restrict access to the postfix sendmail command?