Hi,
not sure if this is the best forum/plugin but as a long time ATH user I ran into issues where plain-text version is missing and it struck me that it would be awesome if there was a plugin/feature that would automatically create plain-text version, ideally markdown-formatted) so it would be displayed just fine without need to switch to basic-html.
IMHO it could also be used as another mode and virtually convert all html mails to markdown (i.e. display plain-text by default if present and if not render html->markdown version). There is even a library that does just that: https://github.com/mixmark-io/turndown