JSI Tip 0807. Your Microsoft Exchange Server is unavailable.

If you receive:

   Microsoft Exchange Server
 
      Your Microsoft Exchange Server is unavailable.
 
      You can retry connecting to the server using the network, or
      work offline and connect using dial-up networking.
 
      Network problems are preventing connection to the
      Microsoft Exchange Server computer. Contact your system
      administrator is this condition persists.
 
-or-
 
   Microsoft Exchange Server
 
      The logon credentials supplied were incorrect. Make sure your
      username and domain are correct then type your password again.

you may have set HKEY_LOCAL_MACHINE\System\CurrentControlSet\control\LSA\LMCompatibilityLevel to a 2.

Windows NT supports Windows NT challenge/response and LanManager (LM) challenge/response. The valid data values are:

0 - Send LM and Windows NT authentication (default).
1 - Send Windows NT authentication and LM authentication only if the server requests it.
2 - Never send LM authentication.

Windows NT authenitcation can not be used on clients that support only LM authentication, like W95 and WFW.