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NOTE: CentOS Enterprise Linux is built from the Red Hat Enterprise Linux source code. Other than logo and name changes CentOS Enterprise Linux is compatible with the equivalent Red Hat version. This document applies equally to both Red Hat and CentOS Enterprise Linux.
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 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.
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.
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).
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 (/usr/sbin/sendmail) is the default SMTP program
under Red Hat Enterprise Linux. However, a simpler mail server
application called Postfix (/usr/sbin/postfix) is also available.
There are two primary protocols used by email client
applications to retrieve email from mail servers: the Post Office Protocol (POP)
and the Internet Message Access Protocol
(IMAP).
Unlike SMTP, both of these protocols require connecting clients
to authenticate using a username and password. By default,
passwords for both protocols are passed over the network
unencrypted.
The default POP server under Red Hat Enterprise Linux is
/usr/sbin/ipop3d and is provided by the
imap package. When using a POP server,
email messages are downloaded by email client applications. By
default, most POP email clients are automatically configured to
delete the message on the email server after it has been
successfully transferred, however this setting usually can be
changed.
POP is fully compatible with important Internet messaging
standards, such as Multipurpose Internet Mail
Extensions (MIME), which allow for
email attachments.
POP works best for users who have one system on which to read
email. It also works well for users who do not have a persistent
connection to the Internet or the network containing the mail
server. Unfortunately for those with slow network connections, POP
requires client programs upon authentication to download the entire
content of each message. This can take a long time if any messages
have large attachments.
The most current version of the standard POP protocol is
POP3.
There are, however, a variety of lesser-used POP protocol
variants:
-
APOP — POP3 with MDS
authentication. An encoded hash of the user's password is sent from
the email client to the server rather then sending an unencrypted
password.
-
KPOP — POP3 with Kerberos
authentication. Refer to Chapter 19
Kerberos for more information.
-
RPOP — POP3 with RPOP
authentication. This uses a per-user ID, similar to a password, to
authenticate POP requests. However, this ID is not encrypted, so
RPOP is no more secure than standard POP.
For added security, it is possible to use Secure Socket Layer (SSL)
encryption for client authentication and data transfer sessions.
This can be enabled by using the ipop3s
service or by using the /usr/sbin/stunnel
program. Refer to Section 11.5.1 Securing
Communication for more information.
The default IMAP server under Red Hat Enterprise Linux is
/usr/sbin/imapd and is provided by the
imap package. When using an IMAP mail
server, email messages remain on the server where users can read or
delete them. IMAP also allows client applications to create,
rename, or delete mail directories on the server to organize and
store email.
IMAP is particularly useful for those who access their email
using multiple machines. The protocol is also convenient for users
connecting to the mail server via a slow connection, because only
the email header information is downloaded for messages until
opened, saving bandwidth. The user also has the ability to delete
messages without viewing or downloading them.
For convenience, IMAP client applications are capable of caching
copies of messages locally, so the user can browse previously read
messages when not directly connected to the IMAP server.
IMAP, like POP, is fully compatible with important Internet
messaging standards, such as MIME, which allow for email
attachments.
For added security, it is possible to use SSL encryption for client authentication and data
transfer sessions. This can be enabled by using the imaps service, or by using the /usr/sbin/stunnel program. Refer to Section 11.5.1 Securing
Communication for more information.
Other free, as well as commercial, IMAP clients and servers are
available, many of which extend the IMAP protocol and provide
additional functionality. A comprehensive list can be found online
at https://www.imap.org/products/longlist.htm.
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