28.1 Introduction

28.1 Introduction

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.
graphics/28fig01.gif

Users deal with a user agent, of which there are a multitude to choose from. Popular user agents for Unix include MH, Berkeley Mail, Elm, and Mush.

The exchange of mail using TCP is performed by a message transfer agent (MTA). The most common MTA for Unix systems is Sendmail. Users normally don't deal with the MTA. It is the responsibility of the system administrator to set up the local MTA. Users often have a choice, however, for their user agent.

This chapter examines the exchange of electronic mail between the two MTAs using TCP. We do not look at the operation or design of user agents.

RFC 821 [Postel 1982] specifies the SMTP protocol. This is how two MTAs communicate with each other across a single TCP connection. RFC 822 [Crocker 1982] specifies the format of the electronic mail message that is transmitted using RFC 821 between the two MTAs.



TCP.IP Illustrated, Volume 1. The Protocols
TCP/IP Illustrated, Vol. 1: The Protocols (Addison-Wesley Professional Computing Series)
ISBN: 0201633469
EAN: 2147483647
Year: 1993
Pages: 378

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