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

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:

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

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

AFAIK they recommend using a dedicated chatmail-enabled provider to avoid problems with using a "normal" email provider.