Setting up the sieve rule means I have to have the password and set all of that up.
Using the "mailing list" settings means I don't have to do any of that, don't need to create an inbox, and it doesn't need to be owned by any accounts (and cloudron users with email admin / admin / superadmin can modify as needed).
Screen Shot 2023-12-15 at 5.08.26 PM.png
So I guess I answered my own question 🙂 I'll write it up. for people who ask in the future. "How do I set up email forwarding to external accounts without having to provision Cloudron accounts" == mailing list.
@luckow thanks for the SoGo -- looks like that does take care of everything.