If you create your Mailbox database before the creation of a Client Access array or the installed a Client Access server within the Active Directory site, you’ll need to reconfigure the value of the RPCClientAccessServer property. If no Client Access server exists in the Active Directory site when the Mailbox database is created, the value of the RPCClientAccessServer property will be set to the FQDN of the Mailbox server. To configure the value of the RPCClientAccessServer property, use the following command.
Once this command is ran reset IIS on your CAS server. Wait a few minutes and try Outlook again.