4.2.2 One Middleman

The second approach is to use a single middleman, let's just call that the email server. The sender writes a message, and send that to the email server. The receiver connects to the Internet, and queries the email server, and download any message that is stored in the server.

This approach seems to be the solution, but it has a problem.

What if the network has a problem, and the message cannot be delivered to the email server (from the sender's machine)?

The sender's machine needs to remain connected to the Internet to reattempt email delivery to the server. This, once again, is an unreasonable demand on a user's machine.