Frédéric Wang wrote an interesting article about sending MathML inside HTML-based emails. And since I found his solution of inserting manually the MathML elements in the markup quite painful, I did what I usually do in such cases: I wrote an add-on...

MathML editing in Thunderbird

So this add-on adds a new button to the formatting toolbar of message composition windows (the π button at the right-hand side of the main window), a new menu entry under Insert and both open a dialog allowing to insert/edit MathML through AsciiMathML. Hope you'll find it useful, it's available from here and it's not a 1.0 yet.

Update: now submitted to AMO, feel free to review it !

/* enjoy */