your config.production.json should work as it is identical to the solution here:
https://forum.ghost.org/t/unable-to-invite-new-admin-users-to-account-without-mailgun-setup/56711/11
I had tried it and it is working for me. by default if you set up ghost.example.com cloudron should set up an email account ghost.app@example.com for you, you should use ghost.app@example.com instead of newsletter@example.com.