Using dpkg-reconfigure postfix at a suitable priority and with an appropriate debconf front end installed (always true except in very, very minimal systems) will resolve it.
Re: mail Relay access denied error after Ubuntu 18.04 upgrade (webwork 2.13)
by Siman Wong - Number of replies: 0
I've belatedly come upon this bug. As a unix non-expert I would appreciate a more precise description of how to patch this (quoted from the debian bug report):