RSS feeds can contain entries with content-type `message/rfc2822`, and it was simple enough to patch Thunderbird's RSS viewer to process those with the email viewer back in the day, so you could read your email with your feedreader. I don't know if the Thunderbird team upstreamed any of that or not, but the context at the time was rssemail [1], which comes up every few years, most recently ActivityPub.

[1] http://mengwong.com/rssemail/