Ubuntu Server 18.04 – Postfix 3.3.0 Issues

After doing a release upgrade to Ubuntu Server 18.04, Postfix stopped working with the following messages in the syslog:

Mar 11 09:56:02 servername postfix/master[1046]: warning: process /usr/lib/postfix/sbin/smtpd pid 12628 exit status 1
Mar 11 09:56:02 servername postfix/master[1046]: warning: /usr/lib/postfix/sbin/smtpd: bad command startup -- throttling
Mar 11 09:57:02 servername postfix/smtpd[12650]: fatal: in parameter smtpd_relay_restrictions or smtpd_recipient_restrictions, specify at least one working instance of: reject_unauth_destination, defer_unauth_destination, reject, defer, defer_if_permit or check_relay_domains

The fix:

postconf -e 'compatibility_level=2'
systemctl restart postfix.service