Environment: 1x Exchange 2013 Mailbox server
1x Exchange 2013 CAS server
All users migrated to office365. MX record pointed to Office365
DIRSync implemented
Clarification: All users are now using Office365. As per recommendation from Microsoft there should be 1 exchange server to be retained and the rest can be decommissioned. I tried to test the scenario by shutting down the exchange server with the mailbox role and leaving the exchange server with CAS role online. I tried to run Exchange Management shell on the CAS but I'm getting errors. To clarify, once I have uninstalled the Exchange server mailbox will the CAS still look for the mailbox server? Or do I need to decommission both Exchange servers and then install a new Exchange server with CAS role?