I don’t know all the places that this may touch, nor how to write tests for this, but I’m posting here so that someone who is more familiar may be able to make this work?
In the user notifications, an argument would be needed to be added that the user’s email should be used in the cc field, instead of to, and set to to some other bucket. The argument would need to be looked for in the message builder to do this.
I think that only adding this argument on L143 of the user notifications would mean this would only affect mailing-list-like emails, and not any other notification, which should still be use the to address.