otherwise mail received via TCP (localhost port 25) is not accepted, since
there is no default smptd_relay_restriction set, which would make the system an
open relay.
in compatibility_level 2 the default is sensibly set to:
'permit_mynetworks, permit_sasl_authenticated, defer_unauth_destination'
reported via the pve-user list:
https://pve.proxmox.com/pipermail/pve-user/2020-February/171368.html
Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>
inet_interfaces = loopback-only
recipient_delimiter = +
+compatibility_level = 2
+
EOD
# produce apt compatible filenames (/var/lib/apt/lists)