It offers three markdown mail composition options: Markdown (text/markdown), Markdown as Plain Text (text/plain), Markdown as HTML (text/plain and text/html). Doesn't seem to be a way to have text/markdown and text/html though.
It also renders incoming text/markdown emails in a HTML-like way using WebKit, rather than showing the raw markdown data.
It also renders incoming text/markdown emails in a HTML-like way using WebKit, rather than showing the raw markdown data.