The actual mail transfer is done through message transfer agents (MTAs). To send mail,
a system must have the client MTA, and to receive mail, a system must have a server
MTA. The formal protocol that defines the MTA client and server in the Internet is called
Simple Mail Transfer Protocol (SMTP). As we said before, two pairs of MTA clientserver
programs are used in the most common situation (fourth scenario). Figure 23.8
shows the range of the SMTP protocol in this scenario.