NT1430

Unit 5
Students search and explore in the following places, finding resources that help them understand mail services in Linux.
ITT Tech Virtual Library
The More Information section of textbook Chapter 20, page 732

Students should use the following keywords and phrases to help them locate appropriate resources:
Sendmail configuration
In order to use Sendmail, first ensure the sendmail package is installed on your system by running, as root: yum install sendmail

In order to configure Sendmail, ensure the sendmail-cf package is installed on your system by running, as root: yum install sendmail-cf

For more information on installing packages with Yum, refer to Section 4.2.4, “Installing Packages”.

Before using Sendmail, the default MTA has to be switched from Postfix. For more information how to switch the default MTA refer to Section 13.3, “Mail Transport Agents”.

The Sendmail executable is /usr/sbin/sendmail.

Sendmail's lengthy and detailed configuration file is /etc/mail/sendmail.cf. Avoid editing the sendmail.cf file directly. To make configuration changes to Sendmail, edit the /etc/mail/sendmail.mc file, back up the original /etc/mail/sendmail.cf, and use the following alternatives to generate a new configuration file:

## Use the included makefile in /etc/mail/ (~]# make all -C /etc/mail/) to create a new /etc/mail/sendmail.cf configuration file. All other generated files in /etc/mail (db files) will be regenerated if needed. The old makemap commands are still usable. The make command will automatically be used by systemctl start|restart|reload sendmail.service.

## Alternatively you may use the m4 macro processor to create a new /etc/mail/sendmail.cf. The m4 macro processor is not installed by default. Before using it to create /etc/mail/sendmail.cf, install the m4 package as root: yum install m4

SMTP in Linux
The Postfix SMTP+LMTP client implements the SMTP and LMTP mail delivery protocols. It processes message delivery requests from the queue manager. Each request specifies a queue file, a sender address, a domain or host to deliver to, and recipient information. This program expects to be run from the master(8) process manager.
The SMTP+LMTP client updates the queue file and marks recipients as finished, or it informs the queue manager that delivery should be tried again at a later time. Delivery status reports are sent to the bounce(8), defer(8) or trace(8) daemon as appropriate.

The SMTP+LMTP client looks up a list of mail exchanger addresses for the destination host, sorts the list by preference, and connects to each listed address until it finds a server that responds.

When a server is not reachable, or when mail delivery fails due to a recoverable error condition, the SMTP+LMTP client will try to deliver the mail to an alternate host.

After a successful mail transaction, a connection may be saved to the scache(8) connection cache server, so that it may be used by any SMTP+LMTP client for a subsequent transaction.

By default, connection caching is enabled temporarily for destinations that have a high volume of mail in the active queue. Session caching can be enabled permanently for specific destinations.
Dovecot
Dovecot is an open-source IMAP and POP3 server for Linux/UNIX-like systems, written primarily with security in mind. Timo Sirainen originated Dovecot and first released it in July 2002. Dovecot developers primarily aim to produce a lightweight, fast and easy-to-set-up open-source mailserver.

Email servers in Linux
The two most widely used protocols on the Internet are http, hypertext transfer protocol (ie. the WWW) and smtp, simple mail transfer protocol (ie. email).

The bulk of the behind-the-scenes email tasks are carried out by the MTA or mail transfer agent. This is the primary software working on a machine set up to be an email server.
Your email client software (Evolution, Kmail, Mozilla's mail client, etc.) will send your email message to the MTA which will then send it out into the Internet and to its intended recipient. So, effectively, the MTA transfers your email message to another MTA, the one that handles the account of its recipient, which then stores it in a file known as a mailbox or a mail spool. Your recipient's client software will then request messages from the his/her server and the mail spool's contents will be transferred to his/her client's mailbox. That is essentially how email works in a nutshell. There can be other programs mixed in there as well.

Fedora 15 email server
Fedora offers two primary MTAs: Postfix and Sendmail. Postfix is configured as the default MTA, although it is easy to switch the default MTA to Sendmail. To switch the default MTA to Sendmail, as root, you can either uninstall Postfix or use the following command to switch to Sendmail: alternatives --config mta

You can also use the following command to enable/disable the desired service: systemctl enable|disable service.service

Postfix
Originally developed at IBM by security expert and programmer Wietse Venema, Postfix is a Sendmail-compatible MTA that is designed to be secure, fast, and easy to configure.

Sendmail
Sendmail's core purpose, like other MTAs, is to safely transfer email among hosts, usually using the SMTP protocol. However, Sendmail is highly configurable, allowing control over almost every aspect of how email is handled, including the protocol used. Many system administrators elect to use Sendmail as their MTA due to its power and scalability.

Grading: The student earns a passing grade for presenting written outlines pertinent to the topic or resources assigned. The student will not earn any grade for this task if no evidence of proper preparation on the topic(s) assigned is available at the time of presentation.