Nice to see this topic has come back to life.

I've been playing with my old org-html-mail.el file, and come up with a
much simpler solution, which takes advantage of the mml message mode
functionality with is used in gnus (and I would imagine in some other
Emacs mail clients, but I can't be sure).

Just call this function and either the active region of your message
buffer or the entire body (if no region is active) will be exported to
html using org-mode, and will be wrapped in the appropriate mml wrapper
to be sent as the appropriate mime type.

So for example this
|            1 |      2 |     3 |
|--------------+--------+-------|
| first column | second | third |

will be exported as this