15.1 Email Architecture

The basic idea of the email architecture (see Figure 15.1) on the Internet dates back to the mid seventies. Nowadays, the standard RFC 821 from 1982 is the basis of the mail communication on the Internet. (RFC 822 describes the form of an email message.) At that time, users were sitting at terminals from which they started mail clients. A mail client has nothing in common with network communication. In essence, a mail client is only a specialized text editor. This text editor can display to the user the message content from the mailbox; it can also work with the messages in the mailbox. It can also do the same with the user’s private mailboxes. Furthermore, it is possible to receive and send messages by means of the mail client. Sending a message does not mean any network communication, only storing the message into a message queue.

The queue of messages is regularly scanned by the SMTP client, which establishes a connection with the remote SMTP server, to...