This is the right question and I won't pretend it's solved yet. Interoperability with SMTP isn't built in currently. The realistic path is a bidirectional SMTP

This is the right question and I won't pretend it's solved yet. Interoperability with SMTP isn't built in currently. The realistic path is a bidirectional SMTP bridge: your FLUX server accepts inbound SMTP from Exchange, Gmail, etc. and converts it to a FLUX envelope on arrival. Outbound works the other way, FLUX messages destined for a non-FLUX address get handed off to an SMTP relay. You'd get the security and features of FLUX within the network, and graceful degradation to regular email outside of it. That's on the roadmap but not built yet. You're right that without it, adoption is a chicken-and-egg problem. Nobody migrates to a protocol they can't use to reach the people they already email.