Email; Email Protocols - Red Hat ENTERPRISE LINUX 3 Reference Manual

Chapter 11.

Email

The birth of electronic mail (email) occurred in the early 1960s. The mailbox was a file in a user's
home directory that was readable only by that user. Primitive mail applications appended new text
messages to the bottom of the file, making the user had to wade through the constantly growing file to
find any particular message. This system was only capable of sending messages to users on the same
system.
The first network transfer of an electronic mail message file took place in 1971 when a computer
engineer named Ray Tomlinson sent a test message between two machines via ARPANET — the
precursor to the Internet. Communication via email soon became very popular, comprising 75 percent
of ARPANET's traffic in less than two years.
Today, email systems based on standardized network protocols have evolved into some of the most
widely used services on the Internet. Red Hat Enterprise Linux offers many advanced applications to
serve and access email.
This chapter reviews modern email protocols in use today and some of the programs designed to send
and receive email.

11.1. Email Protocols

Today, email is delivered using a client/server architecture. An email message is created using a mail
client program. This program then sends the message to a server. The server then forwards the message
to the recipient's email server, where the message is then supplied to the recipient's email client.
To enable this process, a variety of standard network protocols allow different machines, often running
different operating systems and using different email programs, to send and receive email.
The following protocols discussed are the most commonly used in the transfer of email.
11.1.1. Mail Transport Protocols
Mail delivery from a client application to the server, and from an originating server to the destination
server, is handled by the Simple Mail Transfer Protocol (SMTP) .
11.1.1.1. SMTP
The primary purpose of SMTP is to transfer email between mail servers. However, it is critical for
email clients as well. To send email, the client sends the message to an outgoing mail server, which
in turn contacts the destination mail server for delivery. For this reason, it is necessary to specify an
SMTP server when configuring an email client.
Under Red Hat Enterprise Linux, a user can configure an SMTP server on the local machine to handle
mail delivery. However, it is also possible to configure remote SMTP servers for outgoing mail.
One important point to make about the SMTP protocol is that it does not require authentication. This
allows anyone on the Internet to send email to anyone else or even to large groups of people. It is this
characteristic of SMTP that makes junk email or spam possible. Modern SMTP servers attempt to
minimize this behavior by allowing only known hosts access to the SMTP server. Those servers that
do not impose such restrictions are called open relay servers.
By default, Sendmail ( ) is the default SMTP program under Red Hat Enter-
/usr/sbin/sendmail
prise Linux. However, a simpler mail server application called Postfix ( ) is also
/usr/sbin/postfix
available.