As part of a project for a client, I needed to do a MIME multipart email which would include both a plain text and HTML version. The HTML version was supposed to be the default version. However, I discovered that the MIME specifaction is somwhat counter intuitive. You must put your prefered version as low as possible and list other versions in ascending order of prefernce. So in my case text/html went below text/plain. I might try to put on exmaple up to explain things better.