Electronic mail (e-mail) is undoubtedly one of the most popular applications. [Caceres et al. 1991] shows that about one-half of all TCP connections are for the Simple Mail Transfer Protocol, SMTP. (On a byte count basis, FTP connections carry more data.) [Paxson 1993] found that the average mail message contains around 1500 bytes of data, but some messages contain megabytes of data, because electronic mail is sometimes used to send files.
Figure 28.1 shows an outline of e-mail exchange using TCP/IP.
Figure 28.1. Outline of Internet electronic mail.
Users deal with a user agent, of which there ...