At some point we'll have to look into what html messages we want to support properly and how we can detect the rest.
I'll start collecting some points here gathered from experience:

• The Qt Richtext support doesn't display a bar next to blockquotes, which makes html replies rather unreadable
• HTML mails often supply their own styling for quoted text, which looks silly.
• The conversation view becomes a bit awkward with html replies and top posting because you have the previous parts of the conversation above and below.

If we ever want to render somewhat readable mail we just might have to take apart to html messages so we can render only what's relevant.