There are plenty of email users on IMAP, and they use web mail to host mail storage. The IMAP clients can do S/MIME (or PGP I suppose).
The bigger problem is trustworthy user discovery service i.e. a directory to exchange public keys. This exists at an enterprise level (active directory) but not globally.
Hi jaggs, I work in Gmail. Yes a lot of lurk on HN. I don't understand your "Send as" flow. Are you sending email to some 3rd party owned account? Usually that sends as SMTP MSA (login/password), so that should work. Is it broken when someone replies?
The bigger problem is trustworthy user discovery service i.e. a directory to exchange public keys. This exists at an enterprise level (active directory) but not globally.