[int_19h]

Making communications digital doesn't mean that they necessarily rely on the Internet. For local dependencies, you can just as well use a local intranet, which could even be a decentralized mesh network - and the use of common protocols like TCP would allow for something like http://www.broadband-hamnet.org/ to be nearly a drop-in replacement for the whole stack above.

And for dependencies that aren't local to begin with, mail doesn't offer any advantages in terms of cascade failure - you're still relying on some centralized infrastructure that can't really be maintained just locally, and if it goes, your connectivity breaks down all the same.