The current model for Deltachat is “do not trust the server at all, trust a direct connection between the endusers” (currently, in-person QRcode scan or nonce invite link).
Conceptually, I really like this method; I very much want DC to retain it. But practically, I can talk to almost everyone I know by conventional email, and I can only talk to three people over Deltachat, due to assorted blockers. A fully-automated, zero-effort way to establish E2EE mail would encrypt more of my communications even if it was compromised and completely useless 80% of the time.
I can’t think of a good way to fully automate key exchange without trusting a third party or a group of third parties (with mutual checking). I believe I’m already trusting the mailservers with my mail metadata (and webmail servers are always a MITM); I’d be willing to trust them to do an initial key exchange. Autocrypt’s trust-on-first-use strategy of just sending the public key in an Autocrypt header would also work. A group of well-reputed public keyservers works. Multiple routes could crosscheck.
And then I’d check the provided keys in-person or via other channels whenever I could. The more mailclients supported such checks (ideally with automated discrepancy reporting), the easier it would get. Some human-readable key hashes could provide additional checks.
But this is all inexpert suggestions at solutions; I might be better off posting problems at 🚧 Adoption Blockers: What is the most anoying missing Feature to you? - #52 by adbenitez
(and I still can’t figure out how to add a contact from a vcard 
)