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).

POP

The default POP server under Community 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 "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 "Securing Communication" for more information.

IMAP

The default IMAP server under Community 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 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 "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 http://www.imap.org/products/longlist.htm.

Dovecot

The imap-login and pop3-login daemons which implement the IMAP and POP3 protocols are included in the dovecot package. The use of IMAP and POP is configured through dovecot; by default dovecot runs only IMAP. To configure dovecot to use POP:

Edit /etc/dovecot.conf to have the line:

protocols = imap imaps pop3 pop3s

Make that change operational for the current session by running the command:
```
service dovecot restart
```
Make that change operational after the next reboot by running the command:
```
chkconfig dovecot on
```
Please note that dovecot only reports that it started the IMAP server, but also starts the POP3 server.
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.
To configure SSL on dovecot:
- Edit the dovecot configuration file /etc/pki/dovecot/dovecot-openssl.conf as you prefer. However in a typical installation, this file does not require modification.
Rename, move or delete the files /etc/pki/dovecot/certs/dovecot.pem and /etc/pki/dovecot/private/dovecot.pem.
Execute the /usr/share/doc/dovecot-1.0/examples/mkcert.sh script which creates the dovecot self signed certificates. The certificates are copied in the /etc/pki/dovecot/certs and /etc/pki/dovecot/private directories. To implement the changes, restart dovecot (/sbin/service dovecot restart).

More details on dovecot can be found online at http://www.dovecot.org.