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.
The `smtp_mynetworks` variable expects a list. Setting it to a string
resulted in each character in the string being interpreted as an item in
the list.
The Zabbix server also serves an SMTP relay, to minimize reliance on
external services when sending notifications. Since it inherits
configuration of the relay from the general *smtp-relay* group, it ends
up allowing all hosts to relay off of it. To avoid this, we set
`smtp_rmynetworks` at the *zabbix-server* group level to only allow the
local machine to relay messages.
