Second question - I would like to enable it by default for all new registered users (and set it for all past users), obviously, they need to be able to toggle it off if they don’t want it, but I’d prefer people were getting email updates after registering.
These are called Digests in the admin, or Activity Summary/Summary emails in the users’ preferences.
They are a collection of top topics from your forum that users receive by email when they don’t visit the forum regularly.