Outlook Express won't connect

We have eight desktop systems in the office, and all but one are working fine. One system just will not connect to our mail server. When starting, I get an error window that states something to effect that 'User name not valid.....'.I've checking all the settings and they are in the same format as the units that are working. If I put in my settings on that computer, I get the same error message for my settings. If I put his settings on my computer, the program accepts all without a hitch, and we can retrieve his mail on my system. Seems to be something unique to the one computer, not the settings.