Or we could perhaps just assume that everyone is fine with having emails appear as being in markdown.
I suspect this is unlikely to upset anyone; Markdown is largely based on conventions that appeared in e-mail in the first place.
[Gruber][1] says: “While Markdown’s syntax has been influenced by several existing text-to-HTML filters, the single biggest source of inspiration for Markdown’s syntax is the format of plain text email.”
Probably he more specifically means setext and Tidbits, though it's used to various extents elsewhere.
http://en.wikipedia.org/wiki/Setext http://docutils.sourceforge.net/mirror/setext.html
Tidbits still uses setext for it's newsletter, and if you look at the two you'll notice a lot of similarities in design.