This looks very good, but I'm not sure piggybacking on email is a good idea, considering the amount of metadata exposed by email headers that cannot be encrypted and are easy to monitor by listening in on SMTP port 25. Not to mention attachments which are likely not encrypted but just encoded in base64.
I've been using DeltaChat for quite a while, and it works like a charm.<p>At one point I had to switch email provider, though: I had joined a small group chat (~10 participants) in DeltaChat, and although we didn't chat a lot, the number of emails and receipt-requests (and RR returns) hit the max number of outgoing emails per day.