Possibly I am missing something, but I think there are two broader restrictions (from my limited experience, on Deltatouch).
- It seems I can’t use a Chatmail account to send an e-mail to a new contact using a non-Deltachat client, not even if I have both their e-mail address and their public key. (I think that if they sent me an e-mail, I could reply).
- I tried to kludge this by using my Chatmail account in another client to initiate the exchange. But while I can export my keys, I can’t figure out how to access the password I use to login to the Chatmail server.
So Chatmail accounts can only really be used on Deltachat clients. They can also only initiate conversations with other Deltachat clients, using the invite links/QR scanning. If the person using another client manages to manually import the key and send an E2EE e-mail to the Chatmail address (beyond the abilities of many), the Chatmail address may be able to reply.
This scenario is becoming more common. Email servers are increasingly publishing public keys for all the accounts they host (which would be more securely done on a keyserver mirrored across multiple domains, but progress-not-perfection). Clients are also increasingly automatically encrypting replies to encrypted emails. This is good! It means I can have E2EE conversations with people who cannot manually configure PGP.
It would be sensible, if a Deltachat user enters (for example) “user.email@proton.me” as a new recipient e-mail address, for Deltachat clients to look up the corresponding public key at https://mail-api.proton.me/pks/lookup?op=get&search=user.email@proton.me,* warn the user that they are trusting the website (tho I think Protonmail has DANE support, so with DNSSEC that’s not terrible; the DANE compliance of the server could be reported in the warning), and then letting the send them E2EE email.