Exchange Server Login Issue

I have administrative responsibility for a Windows 2000 Server that has Exchange 2000 running on it. It has wirked perfect for over a year. When folks arrived this morning and opened Outlook to access e-mail etc they were asked for their user ID, Password and Domain. The Echenage server is operational and can be accessed via the Outlook Internet client with the same ID and Password they use from the desktop. I have discussed this with another Admin and we are at a loss as to waht has happened.

Can anyone help?

Thanks
BFord