I vote for SIMPLE SIMPLE SIMPLE and let the non-critical extensions be
included in another document.
I agree. And I like the basic idea.
I would prefer if richmail was just SGML. I think most unmarkedup
documents are valid SGML. So the user should be able to put in a few
markup commands and get more pleasing results. In this case a filter,
such as yours, that removes markup is nice to have. Equally nice is a
filter that maps either your richmail markup or my SGML markup to
other similar schemes, e.g. LaTeX.