Google Delta Chat. It’s a chat protocol/server/client based on email. You only need the client - it will use your existing email account (and your recipients’);

Google Delta Chat. It’s a chat protocol/server/client based on email. You only need the client - it will use your existing email account (and your recipients’); if you use the Delta client, it will appear as chat; if you don’t it will just be 2 line emails - and similarly for your addresses. When/if the actual server comes of age, you’ll have additional features like online indicators and stuff. Bit it’s already usable today (if somewhat wasteful in bandwidth and storage compared to e.g. Signal) and there are no new accounts to register or discover — it’s usable with standard email as transport.