Just to clarify on the 64 character key concern: FLUX supports regular human-readable addresses like [email protected] through the account system. The fx1... keypair is the underlying cryptographic identity but you never have to expose it. You'd give your doctor your normal-looking address just like you do today.

The pre-authorized sending idea and the pending folder flow are genuinely good suggestions though, that's something I want to add. And you're right that backwards compatibility is probably the make-or-break problem. A FLUX-to-SMTP gateway is on my list so you can have a flux address that also accepts regular email as a fallback, lowering the barrier to adoption without forcing anyone to abandon their existing setup.