Exchange environment that have many different mailbox server or mix environment with both exchange 2010 and 2013, Some time the outlook get wrong mailbox server. this can happen when you create the mailbox database and the exchange set the cas server by it self.
You can set the CAS server per mailbox database server , to check whitch is the current CAS server for the mailbox database :
Get-MailboxDatabase <Your mailbox database name> | fl RpcClientAccessserver
To set it for the right one :
Set-MailboxDatabase <Your mailbox database name> -RpcClientAccessserver <Full name of your mailbox database server>
Now try Again 🙂