For future visitors: https://github.com/mmanela/MarkdownOutlook
Per their wiki:
A simple plugin for Outlook 2013 which adds a button to the compose mail menu to enable Markdown mode. When enabled your email will be turned from Markdown syntax to html when sending.
- Uses MarkdownSharp for the markdown conversion
- Uses VSTO to integrate with Outlook
- Uses an Installer Project to build an installer