Drop this in your .zshrc/.bashrc, copy some Markdown to the clipboard, and run `mkemail`. Paste the HTML wherever you want. Requires `pandoc` installed (brew install pandoc).

markdown-to-email.sh

# Take a Markdown email from the clipboard or a file and turn it into HTML
function mkemail() {
  # Either read from clipboard (default) or read from a file
  if [[ -z $1 ]]; then
    content=`pbpaste`
    echo "Reading from clipboard"
  else
    content=`cat $1`
    echo "Reading from file '$1'"
  fi

  # Extract the subject from the first line
  subject=$(echo $content | head -n 1 | grep 'Subject:')

  # If there was no subject, convert all of the content
  # Otherwise, strip off the Subject and the blank line that follows
  if [[ -z $subject ]]; then
    echo $content | pandoc -f markdown-smart -t html-smart --wrap=none | sed -e 's/<pre><code>/<pre>/g' -e 's/<\/pre><\/code>/<\/pre>/g' | pbcopy
    echo "HTML copied to clipboard"
  else
    echo $content | tail -n +2 | pandoc -f markdown-smart -t html-smart --wrap=none | sed -e 's/<pre><code>/<pre>/g' -e 's/<\/code><\/pre>/<\/pre>/g' | pbcopy
    echo "Skipped first 2 lines"
    echo "HTML copied to clipboard"
    echo $subject
  fi
}