[e12e]

I generally think /italics/ and bold, quoting "> ", ">> " etc work fine. In addition to utf-8 encoding, you've covered a lot of ground (for English-speakers, utf-8 might seem like a luxury, and of course, if you demand unicode support you're beyond "basic" plain text -- it is however what I mean when I say I prefer plain text emails).

Formulas an illustrations can usually just be appended (and while they won't be shown in-line most clients will display images (and those choosing clients that don't won't really complain), but yeah, if you need multimedia you need multimedia.

Now, I don't really see how an image of an equation is really enough -- if you're working with someone, you'd want them to be able to quote you, reply to you -- and most importantly, tweak your work (edit your equations). I'd argue such (genuinely rich documents) don't really belong in email. Use a wiki or something (and then you can email wiki-markup...).

In short, I'm not convinced all the down sides and added complexity of html mail is worth the hassle.

Are rich documents and hypertext (hypermedia) a good idea? Yes. Does it imply a truly object oriented system, essentially mailing each other runnable smalltalk code? Yes. Will that be secure? No. Will that be standardized? Not by the looks of things. This is essentially why office suites are a source of security holes and incompatibilities. And web apps (though differently).