Clarification: Decommissioning Exchange Mailbox server after move to Office 365 will not cause problems with the remaining Exchange CAS server

Question

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?

Answer 1

Hi
If you are looking for a hybrid coexistence with office 365 then at least one Exchange 2013 Client Access and one
Exchange 2013 Mailbox server must be installed in the on-premises organization to run the Hybrid Configuration wizard and support Exchange 2013-based hybrid deployment functionality.
http://technet.microsoft.com/en-us/library/hh534377(v=exchg.150).aspx
Summary - You need to have at-least one CAS and MBX combined together in onpremise or it can even be seperate CAS and seperate MBX but microsoft recommends to have both CAS and MBX together in onpremise
Source - 
http://technet.microsoft.com/en-us/library/hh534377(v=exchg.150).aspx
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