Is this for a Hotmail or MSN account? If so, then read the following taken
from:
http://help.msn.com/resources/targeted/en-US/HotmailPIMv10/content/pim_trou_nofreedav.htm
Can't access my free account from Microsoft Outlook or Outlook Express
To protect our users from the increasing amount of unsolicited junk e-mail
or spam sent from MSN Hotmail accounts through Outlook and Outlook Express,
we have reluctantly had to restrict the new use of Outlook and Outlook
Express to subscribers only.
By making this feature part of a subscription (which requires a user to
provide billing information), we can more effectively manage and put a stop
to the abuse of this feature, and help protect you and others from
unsolicited mail.
If you purchase a subscription service such as MSN Hotmail Plus, or MSN
Premium, you get access to your account from Outlook or Outlook Express,
along with many other added features.