Appointments, Contacts, and Tasks server side synchronization disabled selection

Hi all,

I have an issue using Microsoft Dynamics CRM OnPremise (server and database version: 9.0.24.8) with activating server side sync of appointments. My configuration is with an onpremise exchange server. Inbound and Outbound email are syncing well but I cannot select the server side sync in the mailbox settings:

As you can see, last select option is completely disabled.

What is the issue?

Thanks for your replies!