Appointments, contacts, and tasks can't be synchronized because the email address of the mailbox is configured with another Microsoft Dynamics CRM organization. The best practice is to overwrite the configuration when you test and enable the mailbox in your primary organization. Also, change the synchronization method for your mailbox in non-primary organizations to None.

To change the primary synchronization organization and overwrite the setting stored in Exchange, click: Settings > Email Configuration > Mailbox > open a mailbox > Test & Enable Mailbox > select Sync items with Exchange from this CRM Organization only, even if Exchanges was set to sync with a different Organization. This will allow server-side synchronization to work for this CRM instance but the other instance would no longer work for synching that mailbox through server-side synchronization. To change the synchronization method for Appointments, Contacts, and Tasks, click: Settings > Email Configuration > Mailbox > open a mailbox > select None for Appointments, Contacts, and Tasks.