I encountered this error when I added a check_sender_access clause to my smtpd_recipient_restrictions in postfixes “main.cf”:
fatal: parameter “smtpd_recipient_restrictions”: specify at least one working instance of: check_relay_domains, reject_unauth_destination, reject, defer or defer_if_permit.
I haven’t really checked, but I am guessing this is because smtpd_recipient_restrictions as the last clause isn’t specifically rejecting all mails that aren’t wanted, and so you risk turning your system into an open relay. I did as the error advised, adding reject_unauth_destination to my restrictions, and all was well.