Whenever I received a message, it was deferred with the following maillog entry:
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
It turned out to be a compatibility level problem. I ignored the main.cf.rpmnew file that came with the package. It contains a compatibility_level parameter.

Having added this and restarted Postfix, everything worked as before.
# postconf -e compatibility_level=2
# systemctl restart postfix
Hope that helps.