possible error messages:

-The connection to the Microsoft Exchange Server is unavailable. Outlook must be online or connected to complete this action.

-Unable to open your default e-mail folders. The information store could not be opened.

-Check to make sure you are connected to the network and are using the proper server and mailbox name. The connection to the Microsoft Exchange Server is unavailable. Outlook must be online or connected to complete this action.

More info :
Outlook connection issues with Exchange mailboxes caused by the RPC encryption requirement