Relay for unknown domain users

Here are working settings:

Following command is enough to enter the Nextcloud environment, bash isn’t needed, see also Howto manage or customize NS8 podman containers

runagent -m nextcloud8

The smarthost.env is filled with data from the mail notification settings.

See also Email notifications — NS8 documentation

Wow, 6 years ago, time flies

EDIT:

Maybe there’s an entry in the mail app logs?