We are on Exchange 2007 and Outlook 2007. Starting about two weeks ago, some Outlook users started getting prompted for their username and password, but Outlook would not accept their credentials. We discovered the cause was that the prefix "MSSTD:" in the proxy FQDN in the Outlook Anywhere settings was missing. Putting it back in fixes it. The problem is, with some people (including me) it is disappearing repeatedly. What could be causing that? Our SSL certificates haven't been changed recently.