By default, Postfix sets smtp_send_xforward_command value to "no". If the value is changed to "yes", Postfix will try to forward the original client name and IP address instead of the SMTP proxy IP through the after-filter smtpd process, so that filtered mail is logged with the real client name IP address.
This might cause Postfix to get a wrong source IP and cause IP Filtering not to function properly.
It is suggested not to change the default setting for this parameter.