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.