How to get Users Email address in an Rest Export View for auth users
Access to the user fields is controlled by \Drupal\user\UserAccessControlHandler::checkFieldAccess(). Which returns forbidden() for email except if it's the current user or the user has administer users permission (which you obviously don't want to grant your normal users).

That is called by \Drupal\Core\Entity\EntityAccessControlHandler::fieldAccess(), collects the default access and everything returned by hook_entity_field_access(). You can't directly overrrule a forbidden(), but you can implement hook_entity_field_access_alter() and replace the default access check with one that allows access.