If you try to expand a public folder in Outlook and you receive the following error:

"Cannot expand the folder. Microsoft Exchange is not available. Either there are network problems or the Exchange server is down for maintenance."

or this message when trying to open open an address book from the public folder:

"The address list cannot be displayed. The Contacts folder associated with this address list could not be opened; it may have been moved or deleted, or you do not have permissions. For information on how to remove this folder from the Outlook Address Book, see Microsoft Outlook Help."Cannot expand the folder. Microsoft Exchange is not available. Either there are network problems or the Exchange server is down for maintenance

You will not receive any errors if you view the public folders through OWA, which makes the problem even more confusing.

Resolution: Start the "Microsoft Exchange RPC Client Access Service" on all your mailbox servers. In this case, both the mailbox servers shut off unexpectedly. When they came back online, that service was not started.