It is pretty much a safe bet that, no matter what I *do* want, "show me the HTML that is used internally by Mailmate to render the plain text part of this message, but which does not actually occur in the message in any way" is not it.
I sometimes get mail where the sender has very carefully added an EMPTY plain text alternative. Which I get by default because I tend to prefer plain text to begin with. If I accidentally select "Show HTML Source" instead of "Message Body Parts -> HTML", then... Whoops! I get something that looks like HTML, so I might mistakenly believe there is actually some relationship between it and the HTML message, but which is not actually part of the message. -s