In an ideal world, I would like to write plain Markdown and send an email off like that, with conversion to HTML happening in the background. A suitable Thunderbird addon for this [1] seems abandoned. Formatting HTML is a real struggle for me in Thunderbird's WYSIWYG editor. On the other hand, using plain-text lacks in readability (lists, short links instead of full URLs, headers, quotes, ...).

[1]: https://addons.thunderbird.net/en-US/thunderbird/addon/markd...