About 5 years too late I'm moving all my email from Eudora via IMAP to "the cloud".
It's becoming apparent that Eudora has mangled the email headers, so when I view any old emails I just see the HTML source.
I can work round it by selecting "Edit as new" and then switching the editor to HTML view, but it would be so neat to have a "View as HTML" button on the mail viewer pane - the way that mail2web.com works.
RoundCube will automatically display the html version of the email if its available so it must how the html in the plan text view. You could write a plugin that would manually show it as html instead of plan text.