@nicfab
FYI That question was primarily aimed at Yunohost / Nextcloud / Freedombox, as I'd love to know if the tools I already use, make this user friendly to setup yet. ;)
Either way, this looks fascinating! While I lack the technical expertise to validate the approach, the promise is extremely appealing, raising the possibility that the most popular, #FOSS social network on the planet (email) could finally be made encrypted for the masses (such as I), doing for email what Let's Encrypt did for web servers.
Coming more from a marketing background myself, my first thought was what it would take to get widespread adoption, which seems to be support from major (FOSS + commercial) email server applications. From a quick skim of your article, it seems adding support shouldn't be onerous...