@fefrei i guess security is the reason you guys put smtp settings in app.yml.
If that 's true, i would simplified that design and put an use-and-forget ‘secret-key’ in app.yml, the first access user must provide this secret key to access the register page(only one person get this key, and it will never be used agaign). And before actual register, this user must provide the system smtp settings, these settings will be saved just like all the other settings in admin panel.