/etc/postfix/master.cf Changes Overwritten

Managed tto get ClamAV virus scanner working under 10.3 Server with a few changes to the instructions at https://www.afp548.com/Articles/mail/postfix-sa.html. However - the installation involves adding some lines to /etc/postfix/master.cf. After restarting postfix the changes are soon deleted - it appears that this file is being overwritten (by Apple?). Anyone else experienced this?