We have Exchange 2003 and Outlook 2007 clients. We have been set up this way for almost nine months now, suddenly last week some of our users have been getting prompted for their Exchange username and passwords and in no particualar pattern. Somedays users X,Y, and Z need to input their credentials and other days they are fine and it's A, B, and C that need the credentials. What would cause this to happen all of the sudden? Our Exchange server is also our DC, and so far there hasn't been any issues with authentication into the domain.