The Zabbix server resolves *localhost* to `::1`, but Postfix resolves it
to `127.0.0.1`. This causes Postfix to reject incoming mail from Zabbix
with "Relay access denied." Explicitly setting the `mynetworks` setting
to include both the IPv4 and IPv6 loopback addresses will ensure that no
mail is rejected from local processes, regardless of how name resolution
happens.