How do I enable RPost for all newly created users?

1) In order to have Registered Mail enabled for all newly created users

then you would need to add the feature to your enterprise default plan.

2) To set your enterprise company default plan please follow this link:

Upon activation of this feature in the enterprise defaults section, you will see a pop up window that states that it will take 24 hours to for the desired action to take place and once the account has been activated or deactivated and email will be sent to the admin to inform them of this action.