We recently migrated users from Exchange 2010 to 2013 and now we have occasional issues with Outlook connections.
From time to time a user will get an error from Outlook stating that they cannot expand a folder from an "additional mailbox, or even their own archive mailbox directory.
This is an example of one of the errors that we receive.
"cannot expand the folder. The set of folders cannot be opened. Microsoft Exchange is not available. Either there are network problems or the Exchange server is down for maintenance."