I’m pretty sure I’ve seen an open source project that uses email as an encrypted chat app in the last months.

Edit: this one https://news.ycombinator.com/item?id=44335065

Interesting - thanks. It looks like at least the Android and iOS apps have been updated in the last few days - so it is under active development:

https://github.com/deltachat/deltachat-android https://github.com/deltachat/deltachat-ios

Doesn't work with all email providers though, from their FAQ:

https://providers.delta.chat/

Proton mail isn't supported (I'm guessing because of the way Proton encrypts your email at rest?).