How E-mail Works
To understand how an MTA works and how to deploy it, you must first
understand how the e-mail process itself works.
The first thing you must understand is the fact that an e-mail system
is composed of several modular components. Each component plays a
different role in delivering messages from the sender to the
recipient. The following steps occur as an e-mail message is sent from
the sender to the recipient.
These are shown in Figure 16-13.
The user composes a message using an e-mail client. The e-mail client
software is called the mail user agent (MUA). Common MUAs include
Evolution, Mozilla Thunderbird, and Mutt. The key thing to remember is
that the MUA has no idea how to get the message to the final
recipient. Instead, it’s configured to send outgoing messages to
a mail server.
When the user selects Send, the MUA sends outgoing messages to a
configured e-mail mail server using the Simple Mail Transfer Protocol
(SMTP). SMTP is an upper-layer, IP-based protocol that runs on port
25. The sole purpose of SMTP is to transfer e-mail messages between
e-mail agents.
The message is transferred from the MUA to the mail transfer agent
(MTA) on the mail server. The MTA then decides what to do with the
message. If it’s intended for another user in the same organization,
the message is held by the local MTA for delivery to the recipient’s
MUA. However, if it’s intended for a recipient in a different domain,
the MTA forwards the message via SMTP through the Internet to the MTA
used by the message’s recipient.
The destination MTA transfers the e-mail message to the message
delivery agent (MDA, also called the Message Store) for the system
where the recipient resides. The MDA’s job is to store the message
until the MUA of the recipient requests it.
The recipient uses e-mail client software (another MUA) to connect to
the Message Store and pulls down the message. To do this, it can use
either the Post Office Protocol (POP) version 3 or the Internet
Message Access Protocol (IMAP). Both protocols are supported by most
MDAs and MUAs. POP3 downloads the message using port 110, and the
message is deleted from the Message Store. IMAP does the same thing
over port 143. However, IMAP has several advantages over POP3. First,
IMAP allows you to download the entire message or only the message
headers. IMAP also allows you to keep a copy of your messages in the
Message Store. In addition, it allows you to create custom folders in
the Message Store for organizing messages.
Most Linux MTAs do not include any POP3 or IMAP functionality. To use
most Linux MTAs with end-user MUAs, you must install and configure a
POP3/IMAP daemon that grabs messages from the MTA and delivers them
via the appropriate protocol to the MUA.
All Linux distributions include an MTA that runs by default on the
system. Several MTAs are commonly implemented on Linux:
• sendmail
The sendmail daemon is the oldest MTA used on Linux, but
it’s still the default MTA used by many distributions. It’s been
widely used for many years. The key thing to remember about sendmail
is the fact that it’s noncomponentized; instead, it’s a single
executable. However, when compared with other Linux MTAs, sendmail can
be somewhat difficult to configure.
• postfix
The postfix daemon is a popular MTA that has replaced
sendmail on many Linux distributions. One of the key advantages of
postfix over sendmail is the fact that it is modular, meaning that it
is composed of multiple programs instead of one single executable.
Each program has its own tasks that it is responsible for. It uses the
master daemon in conjunction with multiple worker daemons that are
only started when they are needed. When their tasks are complete, the
master daemon automatically shuts them down. In effect, the master
daemon and the worker daemons function together as a unit to process e-mail
messages through the MTA. The process is
shown in Figure 16-14.
1) If an e-mail message is sent to the Postfix MTA by a remote MTA or
remote MUA, it is accepted into the system by the smtpd worker daemon.
The smtpd daemon forwards the message to the cleanup daemon.
2) If an e-mail message is sent to the Postfix MTA by a user using the
shell prompt on the local system, the following occurs:
a) It is accepted into the system by the postdrop daemon, which is also a
postfix worker daemon.
b)The postdrop daemon places the e-mail message in the maildrop
directory in /var/ spool/postfix.
c) The pickup daemon transfers the message from the maildrop directory
and forwards it to the cleanup daemon.
3) The e-mail message, regardless of whether it originated remotely or on
the local system, is then processed by the cleanup daemon.
4) The cleanup daemon places the e-mail message in the incoming queue,
which is also /var/ spool/postfix.
5) The cleanup daemon notifies the Queue Manager daemon (qmgr) that a
message has been added to the incoming queue.
6) The qmgr daemon moves the e-mail message to one of five message
queues, all of which are located in /var/spool/postfix:
- incoming
This queue contains incoming messages that have been
processed by the cleanup daemon.
- active
This queue contains messages that arrived intact, were
processed correctly, and are ready to be forwarded on to the next MTA
or MDA.
- hold
This queue contains messages whose delivery, for some reason, has
been put on hold.
- deferred
This queue contains messages that could not be delivered on
the first attempt. The qmgr daemon will wait a while; then it will
make several more delivery attempts.
- corrupt
This queue contains messages that were damaged somewhere in
transit and are no longer readable.
7) The qmgr daemon then invokes the trivial-rewrite daemon, which
identifies the next MTA or MDA to which the message should be sent.
8) The qmgr daemon then passes the message to the appropriate delivery
agent. If the message is intended for a local user account, the local
agent is used. If the message is intended for another MTA or an MDA,
the smtp agent is used.
9)The selected agent transfers the e-mail message to the appropriate system:
- qmail
The qmail MTA is also a modular MTA daemon that is frequently
used as a replacement for sendmail. The qmail MTA is unique in that it
was designed from the ground up with security in mind. Essentially,
its modular architecture is composed
of mutually untrusting components. This means each module in the MTA
(such as the SMTP listener) will use a set of credentials that are
unique to it. No two modules use the same credentials. Many
administrators consider qmail, like postfix, easier
to manage than sendmail. However, it’s not the default MTA used by any
Linux distribution because it has components that are covered by a
proprietary licensing agreement. In spite of this, qmail is greatly
loved by those who use it. My niece is married to a system admin, and
he swears by qmail. You need to be aware that two additional mail
protocols are implemented by qmail:
• Quick Mail Queuing Protocol (QMQP)
Allows the sharing of e-mail queues among different MTAs
• Quick Mail Transport Protocol (QMTP)
A transmission protocol similar to SMTP, but considered to be faster
- Exim
For your LPIC-1/Linux+ exam, you also need to be familiar with
the Exim MTA. Exim is a nonmodular MTA, composed of only a single
program (similar to sendmail). However, it is considered to be much
easier to configure and manage than sendmail. A limited number of
Linux distributions use Exim as the default MTA.
As mentioned earlier, these MTAs can be used to receive sent e-mail
messages from MUAs via SMTP. However, they can’t download messages to
e-mail clients via POP3 or IMAP. To do this, you must install the imap
package on your system using the software management utility of your
choice. This package installs IMAP and POP3 daemons that you can use
to transfer messages from your MTA to your e-mail client software.
Both of these daemons are managed using the xinetd daemon. You can
enable these daemons using the /etc/xinetd.d/imap file. Once this is
done, restart the xinetd daemon.
If your distribution uses init, you manage your MTA daemon using the
appropriate init script in your init directory. For example, the
postfix init script is appropriately named postfix, whereas the
sendmail init script is sendmail. If your distribution uses systemd,
you use the systemctl command to start, stop, or reload the
appropriate service file for the MTA installed on your system.
LX0-104 Exam Objectives (P)
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