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