After reading this in the administration guide:
"A user can be a member of more than one Dynamics 365 organization, but an Exchange mailbox (email address) can only synchronize emails, appointments, contacts, and tasks with one organization, and a user that belongs to that organization can only synchronize emails, appointments, contacts, and tasks with one Exchange mailbox."
What is the best practice for managing users and syncing email when a user belongs to multiple orgs within the same instance.