Right now messages are sent as fixed text using quoted printable encoding. Using the Its All Text firefox plugin with a properly configured editor, you can reflow reply text and wrap your own text, but the message is still sent using a fixed encoding.
Add a combo box to the compose form to allow the user to select how the message should be sent. I think for flowing, the easiest is to use pandoc and send an HTML alternative. The other option is to send format=flowed, but for that we would need some way of
- fixed plain: send the body exactly as in the textbox
- fixed quoted printable: send the body using quoted printable
- markdown: parse the message using the excellent pandoc library. Send the raw markdown as a fixed quoted-printable part but then also send the resulting html as a html alternative. Since markdown uses email quotes for blockquotes, this will even parse and reflow quoted sections automatically.
I think these three are enough, but to these three we could add a format=flowed option. I think we would use pandoc to parse the text as markdown and then walk the parsed representation to find the paragraphs and quotes and such.