If I understand correctly, blocking is basically impossible unless a "citizen" is blocked from using any mail server not regulated by a specific country that then demands scanning for it?
I’m more thinking in terms of mailbox-as-identity being a single point of failure leaving you at the mercy of choosing one reliable provider, whereas mailbox-agility means you can migrate transparently. An analogy I would make would be akin to your personal website having your own domain name and hosting on GitHub pages, instead of having your GitHub pages address as your permanent handle.
