Red Hat ENTERPRISE LINUX 5 - VIRTUAL SERVER ADMINISTRATION, Руководство по развертыванию

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Chapter 24. Email
378
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.
24.1.2. Mail Access Protocols
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
).
24.1.2.1. POP
The default POP server under Red Hat Enterprise Linux is
/usr/lib/cyrus-imapd/pop3d
and is
provided by the
cyrus-imapd
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
Section 43.6, “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 24.6.1, “Securing Communication”
for more information.
24.1.2.2. IMAP
The default IMAP server under Red Hat Enterprise Linux is
/usr/lib/cyrus-imapd/imapd
and is
provided by the
cyrus-imapd
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