Mail clients and mail servers

In order to send mail between two nodes, there ideally should be some kind of client/server setup in which the exchange occurs.

The mail client is responsible for obtaining mail from the host mailbox (see next chapter) while the mail server is responsible for receiving incoming mail and storing it in the mailbox so that it may be fetched by an external client.

When transmitting mail, the mail client is to complete a proof of work computation which must have acceptable results in order for transmission to complete.