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 5.1, “Securing Communication”
for
more information.
1.2.2. IMAP
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.
Mail Access Protocols
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