I have not run into this but I get very little HTML formatted mail. Could you forward a message that does not work for you to jason@hastymail.org so I can test it out? If I had to guess I would say maybe the filter white-list is too restrictive and is cutting out most of the message. Or it is a bug :)
Thanks for the feedback,
Jason
Jason, I've bounced you a copy out of my spool.
I'm sorry but I did not receive your email. Could you open a bug report and attach it? You have to sign up but it is free and private. Also this problem may have been solved already in SVN. We had a similar report and corresponding fix shortly after RC8 was released. You can download the latest SVN from the Download page (at the bottom). It's very stable since we are getting close to another release.
Thanks for the feedback,
Jason