On Sept 25, 2019, we will be releasing an opt-in feature that allows managed users to receive Box notifications at a verified notification email address that is different than their primary Box email address. The notification email can be configured by the admin in the Admin Console, by admin/developers through the API, or by the end user in the web application if the admin has set up the enterprise setting to allow end users to change their own emails.
(Note: edited title to clarify this feature is for managed external users)