I'm in the UK so it's late at night here and doesn't take much to confuse
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If all your settings have been entered exactly the same as the other two accounts and it's the same provider, then it's indeed an odd program. Outlook Express is convinced that there is an error with the username or password being used, that's what its reporting back. Make sure you are definitely entering the correct ones. It might sound trivial, but removing the account and recreating it might solve the problem, there could be some tiny inconsistency in the details that is evading you.