After moving mailboxes to the new exchange 2007 or 2010 server and then decommissioning an old exchange 2003 server and attempting to access the public folders this message came up
The set of folders cannot be opened. Microsoft exchange is not available. Either there are network problems all the exchange server is down to maintenance. (/o=First Organisation/ou=First Administrative Group/cn=Configuration/cn=Servers/cn=EXCHANGE
Where EXCHANGE is the old 2003 server name
This occurs because the new mailbox is still associated with the public folders on the old server. You need to have replicated or the public folders to the new server and then change the private mailbox store association to the new server. To do this open exchange management console on the new server.
Under Organisation configuration\mailbox
Right click on the private store file and select Properties
Under the client is settings tab Browse to select the new server and apply the settings
I found the settings did not take straight away so rebooted the exchange server the mailboxes were residing on to initiate the new setting.