Sadly Keith Moore's addrquery draft seems to have stalled:
https://tools.ietf.org/html/draft-moore-email-addrquery-01
Unfortunately, the draft is useless for end-to-end encryption, as it
relies on a clean path from the client to the recipient's SMTP server ...
I would encourage anyone interested in this topic to read the draft,
particularly section 4. No, it does not depend on a clean path from
the MUA to the recipient MTA.
R's,
John