By default Outlook 2003 has encryption disabled, and Exchange 2010 has the requirement enabled. This can cause the Your Microsoft Exchange Server is unavailable error. There are two ways to resolve this.
1. Enable Encryption in Outlook 2003
Encryption is enabled in the Outlook profile. Under the Outlook Profile select More settings > Security and ensure Enable encryption between Outlook and the Exchange Server is ticked.
2. Disable Encryption on the CAS
If you don’t want to make these changes to the Outlook profiles you can turn off the encryption requirement on the Client Access server instead. Outlook clients can still make encrypted connections.
From the Exchange Management Shell run the following command.
Set-RpcClientAccess -Server CAS01 -EncryptionRequired $false
This will remove the requirement for encryption on the CAS, however if public folders are in use it will also need to be removed on the relevant mailbox servers hosting the public folders.
Slow performance in Online Mode
When Outlook opens an initial connection to an Exchange server, it attempts to register itself to receive new message notifications. If Outlook can register for UDP notifications, an Exchange server sends a notification to Outlook by using UDP when a new message arrives in the mailbox. When Outlook receives this UDP notification, Outlook retrieves the message from the Exchange server and displays the message in the appropriate folder. If Outlook does not receive new mail notifications through UDP, Outlook reverts to polling the Exchange server for changes. Although this polling behaviour is initiated by Outlook, the Exchange server dictates the polling frequency. By default, Outlook performs polling every 60 seconds. Exchange Server 2010 does not issue UDP notifications to Outlook, therefore Outlook cannot register to receive the notifications and only receives changes when it polls the server.
Again there are two options
1. Use cached mode (not always an option)
2. Install Exchange 2010 Update Rollup 1 and make the following registry change.
Create the \ParameterSystem subkey if it does not exist
Create a new DWORD value called Maximum Polling Frequency
Enter a value between 5000 and 120000 (5-120 seconds)
Note that this change is dynamic and clients will need to close and re-open Outlook for the change to take effect.