>>> #
>>> # Initialize.
>>> #
>>> #! ../bin/postmap smtpd_check_access
>>> #msg_verbose 1
>>> smtpd_delay_reject 0
OK
>>> mynetworks 127.0.0.0/8,168.100.3.0/28
OK
>>> #
>>> # MX backup
>>> #
>>> mydestination wzv.porcupine.org,localhost.porcupine.org
OK
>>> inet_interfaces 168.100.3.7,127.0.0.1
OK
>>> recipient_restrictions permit_mx_backup,reject
OK
>>> rcpt wietse@wzv.porcupine.org
./smtpd_check: warning: support for restriction "permit_mx_backup" will be removed from Postfix; instead, use "relay_domains"
OK
>>> rcpt wietse@backup.porcupine.org
OK
>>> rcpt wietse@porcupine.org
./smtpd_check: <queue id>: reject: RCPT from localhost[127.0.0.1]: 554 5.7.1 <wietse@porcupine.org>: Recipient address rejected: Access denied; to=<wietse@porcupine.org> proto=SMTP
554 5.7.1 <wietse@porcupine.org>: Recipient address rejected: Access denied
>>> permit_mx_backup_networks 168.100.3.5
OK
>>> rcpt wietse@backup.porcupine.org
./smtpd_check: <queue id>: reject: RCPT from localhost[127.0.0.1]: 554 5.7.1 <wietse@backup.porcupine.org>: Recipient address rejected: Access denied; to=<wietse@backup.porcupine.org> proto=SMTP
554 5.7.1 <wietse@backup.porcupine.org>: Recipient address rejected: Access denied
>>> permit_mx_backup_networks 168.100.3.4
OK
>>> rcpt wietse@backup.porcupine.org
OK