The first and the second stages of mail delivery use SMTP. However, SMTP is not
involved in the third stage because SMTP is a push protocol; it pushes the message
from the client to the server. In other words, the direction of the bulk data (messages) is from the client to the server. On the other hand, the third stage needs a pull protocol; the
client must pull messages from the server. The direction of the bulk data are from the
server to the client. The third stage uses a message access agent.
Currently two message access protocols are available: Post Office Protocol, version 3
(POP3) and Internet Mail Access Protocol, version 4 (IMAP4). Figure 23.13 shows the
position of these two protocols in the most common situation (fourth scenario).