[Focal] Emails not being sent from mon server under Focal

[emkll]

Description

It appears that the monitor server's postfix configuration isincompatible with Ubuntu 20.04 Focal

Part of #4768

Steps to Reproduce

1. Install SecureDrop on a Focal host
2. Observe error in syslog on mon:

mon postfix/smtpd[3103]: 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

Expected Behavior

Emails should be set from the mon server containing ossec alerts

Actual Behavior

Emails are not being sent due to incompatibility in postfix configuration.