Outlook clients may receive an error when they try to connect to mailboxes after the Client Access server role is moved between two Exchange Server 2010 servers.
Cannot start Microsoft Office Outlook. Unable to open the Outlook window. The set of folders could not be opened. The server is not available. Contact your administrator if this condition persists.

Unlike the previous versions of Exchange in Exchange Server 2010 all client communications, including MAPI clients, connect to the RPC Client Access service running on the Client Access server role. Thanks to the componentized server role architecture of Exchange Server 2010 it is possible to move this role between servers to suit the needs of the organization.
For example, an organization may deploy Exchange Server 2010 on a single server, and then later scale that out to separate the Client Access server role to a different server.
Each mailbox database in the organization is associated with an RPC Client Access service in the same AD Site when it is first created. This association is not dynamic. If the Client Access server role is removed from the single Exchange server and deployed to a new server, all of the existing mailbox databases are no longer associated with a valid RPC Client Access service name.
The association can be seen on the mailbox database attributes by running this command in the Exchange Management Shell.
[PS] C:\>Get-MailboxDatabase | fl name,*rpc* Name : Mailbox Database 0447493325 RpcClientAccessServer : ex1.domain.local
If the Client Access server role was moved from ex1.domain.local to ex2.domain.local, you can associate the mailbox database with the new RPC Client Access service by running this command.
[PS] C:\>Set-MailboxDatabase -Identity "Mailbox Database 0447493325" -RpcClientAccessServer ex2.domain.local
You can perform this action for all mailbox databases on the server by running this command.
[PS] C:\>Get-MailboxDatabase -Server ex1.domain.local | Set-MailboxDatabase -RpcClientAccessServer ex2.domain.local




Thanks for the information !!!!
Thank you a million. I spend about 5 hours trying to troubleshoot this issue with Outlook resetting itself to the wrong rpc server after moved a CAS to a different server. This simple command put me back in business.
Thank you!
Does it matter if the server name: ex1.domain.local is using a different FQDN on the outside like: mail.domain.local ?
No, thats fine. It is quite normal to have different internal and external URLs configured.