Either you are entering the wrong username/password, or else that
account is corrupted. One thing you might try as a possible quick fix
is to delete that corrupted account, restart Windows Mail, then
recreate that mail account. If that doesn't fix it, more drastic
measures will have to be taken.
That type of account corruption is often caused by interference from
a non-compatible antivirus program. Which antivirus are you running?
As a minimum, email scanning in the antivirus must be turned off,
although that may not be sufficient to eliminate all bad effects.
In a worst case scenario, your antivirus may need to be uninstalled.
If turning off the email scanning function does not resolve your problem,
consider upgrading to Windows Live Mail, because it is less prone to
suffering bad effects from overly aggressive antivirus programs.
http://get.live.com/wlmail/overview
Email scanning in any antivirus must be disabled, for reasons
explained here:
http://thundercloud.net/infoave/tutorials/email-scanning/index.htm
