After my company installed Microsoft Exchange 2000 Server, we discovered that our Microsoft Outlook 2000 plain text messages sometimes arrived in HTML format. This problem was serious for us because we provide technical support on email lists for visually impaired and blind people, many of whom use text-based email packages with speech synthesizers.
After receiving complaints for 2 or 3 weeks, we learned from a Microsoft source that Exchange 2000 has a bug that causes the program to randomly convert plain text messages to HTML. Our source told us to call Microsoft's premium support service and tell them we have the problem the Microsoft article "XFOR: Exchange 2000 Sends MIME Plain Text as HTML and Plain Text" (http://support.microsoft.com/default.aspx?scid=kb;en-us;q286291) describes.
Microsoft sent us a hotfix that didn't solve the problem. When we called the company again, Microsoft initially told us they would charge us for the call. After a week, we finally spoke with a senior engineer (free of charge) who quickly figured out that in addition to employing the hotfix, we needed to make the registry entry SendPlainTextAsIs and reboot the system. As far as we know, the problem we encountered isn't public knowledge.