[mottosso]

I've been using "Markdown Here" for the past few years and live in this ideal world already. It's great, you should come visit. :)

[diarrhea]

The link in my comment is to "Markdown Here" as a Thunderbird addon, but it is not compatible with recent Thunderbirds.

[mtlynch]

+1 for Markdown-Here. It works well enough for me despite it being abandoned.