This is great and all, but you're relying on IMAP, an inherently mobile-unfriendly protocol. They should have worked with Fastmail to get this into JMAP.
There is a mailing list though. At the very least they might add a 'why not (also) JMAP' bullet point to the website if you ask about it:
> […] join the COI developer mailing list by sending a mail to dev-join@coi-dev.org.
I would guess that they need IMAP to prevent the whole 'nobody uses it' problem.
also was there any tests how long does it take for delivery, because the message could hop around and could be scanned, added addotional headers or mess with those there.