I have not found yet a reason why the newer [resources.system_user] allow_email = true would not work.
+ It does not interfere with (list as) regular user accounts.
+ It’s a configurable app setting: Customizing an app's email-address with new "resources" app-install-system? - #10 by tmb
Sending is authenticated with username and its auto-generated password, and gets dkim signed.
It seems to be set up only for sending by default (‘noreply’ bounce), but users can set it as alias and then mails are accepted (they can send and will receive).
Or do you really know about a still current, or future (e.g. trixie) blocker? Doesn’t seem like mobiizon actually needs an own inbox imap account.