Chapter 15.

Email

Email is one of the most widely used services on the Internet. Red Hat Linux offers many ways to

serve and access email, whether you are a desktop user or a system administrator.

This chapter looks at popular email protocols that are in use today and some of the programs designed

to deal with email.

15.1. Protocols

Email, like other network services, uses a variety of protocols. These protocols allow different ma 

chines, often running different operating systems and using different email programs, to communicate

with one another via email.

The following protocols are the most commonly used to transfer email from system to system.

15.1.1. IMAP

The Internet Message Access Protocol (IMAP) is a method used by email client applications to access

remotely stored messages. When using IMAP email messages remain on the remote mail server where

users can read or delete them and create, rename, or delete mailboxes to store the email.

In addition, IMAP is fully compatible with important Internet messaging standards, such as the Mul 

tipurpose Internet Mail Extensions (MIME), to allow for receiving attachments. Many email clients

that use IMAP can also be configured to cache a copy of the messages locally, so that you can browse

previously read messages when you are not directly connected to the IMAP server.

IMAP is primarily used by those who access their email using multiple machines. Also, users that

connect to the Internet or a private network via a low bandwidth connection often use IMAP because

only the email header information is pulled off at first. This allows them to defer the downloading of

messages containing large attachments until a time when their limited bandwidth is not in use. In the

same way, email that the user does not want can be deleted without viewing the message body, saving

the need to even download it through their network connection.

The Request for Comment (RFC) documents that cover IMAP contain the assorted details and specifics

about how the protocol is designed to work. RFC 1730 first defined the way IMAP is used in version

4, but RFC 2060 discusses the IMAP implementation used with many IMAP servers, called version

IMAP4rev1.

The

imap

package in Red Hat Linux allows users to connect to your system and receive

their email using IMAP. Secure IMAP connections are supported through Secure Socket

Layer (SSL) technology built into the

/usr/sbin/imapd

daemon, allowing it to use the

/usr/share/ssl/certs/imapd.pem

certificate file. The

stunnel

program is not required to

provide SSL encryption for IMAP connections, though it can be used. See Section 15.6.2 for more

information concerning these two encryption options.

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 at

http://www.imap.org/products/longlist.htm.

15.1.2. POP

The Post Office Protocol (POP) allows email clients to download email from remote servers and save

those messages on their local machine. Most POP email clients are automatically configured to delete