Perhaps the first fundamental is that Internet email isn't handled by one kind of server. It's a suite of protocols that are served by distinct processes.

At minimum, you have inbound (POP3, IMAP) and outbound (SMTP) servers. You may also have a separate authentication server (LDAP) and perhaps other pieces providing calendaring, web mail (HTTP), and central storage of client configuration (ACAP).

The different protocols exist not because some programmer thought it would be cool to create a new protocol, but because they serve completely different needs. For example, POP3 was designed to support lightweight disconnected clients. IMAP provides server-based storage of mail folders. LDAP provides authentication not just for mail systems but for many other applications. (I'm sure experts for each of these protocols could provide better explanations. The RFC for each probably has a motivation section that you can quote.)

An important "slide" in the presentation would show the plumbing that ties all these protocols and processes together and shows how mail moves through the system. In some cases the plumbing itself is variable. For example, spam and virus processing might occur within several servers or within a client.