How to send an email with LaTeX

On some browsers you can install an extension called Markdown Here, which is capable of rendering markwdown syntax into HTML in some rich-editing boxes. In particular it works with the Google Mail composing message box.

Admittedly, markdown is not LaTeX, but you get anyway a simple and familiar syntax (it is the one used here at stackexchange) to format your messages. In addition, it supports TeX equations (you surround them with dollars), but the method is sub-optimal, since it calls google to render them (did you know that google can render TeX math into images? I didn't) and paste a link to the resulting image. This feature is disabled by default for privacy issues.

Here is a pair of screenshots of the compose box, first while I write the message:

Then after converting it (Ctrl-Alt-M or toolbar button):

Maybe it's too late to answer now, but what about Thunderbird? Thunderbird is open source software. You just have to install "LaTeX It" (it's an addon in Thunderbird). Hope it helped :-)

Some information about the Addon. But download it directly in Thunderbird addon center! That's easier than installing it by the link.

There is a Chrome extension called TeX for Gmail (available from the Chrome Web Store), which can typeset math expressions, equations, and theorems, among other things. I experimented with it and it has been working for me.