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Linux Operations and
Administration
Chapter Twelve
Configuring a Mail Server
Objectives
• Identify key elements in the Linux e-mail
architecture
• Describe how MTAs and MUAs are used in the
Linux mail system
• Configure a mail server in YaST
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Introduction to the Linux Mail System
• E-mail predates the Internet
• Today, e-mail is the most widely used Internet
service
• OpenSUSE includes sophisticated e-mail programs
– Such as Evolution and Kmail
• In Linux environment, e-mail is modular
– A separate program is used to manage each
function
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Introduction to the Linux Mail System
(cont’d.)
Figure 12-1 The Linux e-mail architecture
© Cengage Learning 2013
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Introduction to the Linux Mail System
(cont’d.)
• Two main types of programs typically used on a
Linux e-mail server:
– Mail transfer agent (MTA)
• A program for receiving and delivering messages via
Simple Mail Transfer Protocol
• In Linux, the MTA is often Postfix or sendmail
– Mail user agent (MUA)
• An e-mail program that enables users to access their
mailboxes for reading and sending electronic
messages
• Examples: Evolution, KMail, command-line utility mail
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Introduction to the Linux Mail System
(cont’d.)
• Simple Mail Transfer Protocol (SMTP)
– TCP/IP protocol that defines how e-mail is sent
across the network
• Postfix
– Default MTA in openSUSE
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Introduction to the Linux Mail System
(cont’d.)
• When MTA receives the message from MUA:
– It determines whether the message is intended for a
recipient on local domain or a recipient on another
domain
– If it’s on the local domain (example.com):
• MTA saves the message in local machine’s message
store until the recipient reads it
– For a recipient on another domain:
• MTA sends the message via SMTP to a remote MTA
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Mail Transfer Agents
• MTA
– Actual mail server
– Responsible for:
• Receiving e-mails from local users
• Forwarding them to a local message store or a remote
MTA
• Messages are transferred from the local message
store to users via mail delivery agent (MDA)
– Protocols which can be used for the transfer:
• Post Office Protocol (POP3)
• Internet Message Access Protocol (IMAP)
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Mail Transfer Agents (cont’d.)
• Process of an MTA sending messages not
belonging to local domain to a remote MTA isn’t
automatic
– Your domain’s DNS settings must be configured
correctly
– MTA needs to be able to establish a connection with
the remote MTA
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Sendmail
• Sendmail
– Most widely used MTA on UNIX/Linux systems
– Not the preferred MTA on openSUSE Linux systems
• Postfix
– Preferred MTA
• Ease of configuration, reliability, and security
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Sendmail (cont’d.)
• In most organizations, employees need to send email to many other domains
• You can edit Sendmail configuration file to:
– Change the MTA behavior to allow multiple domains
– Set rules, such as message filters
• Sendmail is a large program compared with other
mail programs
– It performs all MTA functions
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Qmail
•
•
•
•
•
Written by Dan Bernstein
A fast, modular MTA
Lightweight, secure, and reliable
Designed with security and ease of use in mind
Easier to configure
– Such as allowing multiple domains
• Multiple modules handle qmail’s MTA functions
– Each no bigger than 30 KB
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Postfix
• Developed by Wietse Venema
• Default MTA package on openSUSE Linux
systems
• Designed as a modular MTA
• Easier to configure, and more reliable and secure
than sendmail
• Its modular design enhances security
– Each module has its own set of permissions to follow
the guideline of least privileges
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Mail User Agents
• MUA
– An e-mail program that enables users to send and
read messages stored in their mailboxes
• In the Linux e-mail system:
– Each user has a local mailbox
– Local MTAs send messages to users’ mailboxes
– MUA doesn’t actually receive messages
• It allows users to read messages that have already
been placed in mailboxes
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Mail User Agents (cont’d.)
• MUAs can store messages in one of two ways
– Method 1
• Messages are stored on users’ computers
• Frees up space on the server
– Method 2
• Messages are stored on the mail server
• Users can access them from any computer on the
local domain
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Mail User Agents (cont’d.)
• Table 12-1
– Describes four MUA packages included with
openSUSE
• All these packages can use both methods of storing
messages
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Mail User Agents (cont’d.)
Table 12-1 Common openSUSE Linux MUAs
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Using Mailx
• Example showing how to use mailx
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Using Mailx (cont’d.)
• Example: using mailx to read messages in mailbox:
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Using Mailx (cont’d.)
• Using mail command with no arguments displays
all messages in mailbox
• Table 12-2
– Describes common commands that can be entered
at the interactive prompt
• Activity 12-1: Using the Mailx Program
– Send and display e-mails with the mailx program
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Using Mailx (cont’d.)
Table 12-2 Common mailx commands
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Mail Server Configuration
• Configuration procedures differ depending on the
Linux distribution
• With openSUSE Linux, you can use a mail server
configuration utility in YaST Control Center to
perform configuration tasks, including:
– Enable virus scanning
– Define the type of network connection MTA uses to
determine how e-mail is delivered
– Configure an outbound mail server that’s used if you
have a dial-up connection or aren’t connected to the
Internet
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Mail Server Configuration (cont’d.)
– Define masquerade name for outgoing e-mail
• Useful for companies that prefer using a more
professional name than actual mail server name
– Configure incoming mail options, such as:
•
•
•
•
•
Firewall settings
Root mailbox account
Whether to accept mail from remote servers
Local alias accounts
Virtual domains
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The Mail Server Configuration Wizard
• Root user can start Mail Server Configuration
Wizard
• yast2 mail command is used to start wizard
• Following sections describe the steps in wizard
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General Settings
• Can use General Settings window to specify
network connection type
– Determines how MTA delivers messages
– Default option is “Permanent”
• Selecting this option allows Postfix (the MTA) to send
messages in real time as they’re created
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General Settings (cont’d.)
– Dial-up option
• For Linux machines that connect to the Internet via a
dial-up connection
• Forces users to queue outbound mail server manually
when they’re ready to send or receive messages
• Example:
– To e-mail a user on a remote computer, you must
use sendmail -q command after sending your
message
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General Settings (cont’d.)
– “No connection” option
• For Linux machines that don’t have access to the
Internet
• Users with this option can e-mail each other only on
local machines
• Can also use General Settings window to enable
virus scanning with AMaViS program
– AMaViS stands for A Mail Virus Scanner
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General Settings (cont’d.)
Figure 12-2 The General
Settings window
© Cengage Learning 2013
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Outgoing Mail
• Outgoing Mail window is used to configure dial-up
connections by:
– Adding your ISP’s SMTP server, such as
smtp.provider.com
– Clicking Authentication button to enter authentication
credentials
• MTAs can send messages to remote MTAs
– If your mail server is connected to the Internet, you
can leave outgoing mail server entry blank
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Outgoing Mail (cont’d.)
• When you send e-mails, your full computer name is
used in the name field
– Example:
• [email protected]
• Most companies prefer to have the organization
name displayed
– To do this:
• Click the Masquerading button
• Set a display name for each user on the network
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Outgoing Mail (cont’d.)
Figure 12-3 The Outgoing Mail window
© Cengage Learning 2013
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Incoming Mail
• Accept remote SMTP connections option
– This check box is disabled if:
• If your computer isn’t directly connected to the Internet
• You have a dial-up connection
– Select this option if:
• If you’re connected to the Internet directly
– If you select this option, you must configure firewall
settings
• Need to select Open Port in Firewall check box
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Incoming Mail (cont’d.)
Figure 12-4 The Incoming Mail
window
© Cengage Learning 2013
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Incoming Mail (cont’d.)
• Downloading section
– If a company hosts e-mail on a server that isn’t
connected directly to its networks:
• Enter remote server’s credentials in Downloading
section
– Protocols that can be used to transfer e-mail from a
remote MTA:
• POP3—Post Office Protocol 3 (POP3)
• IMAP—Internet Message Access Protocol (IMAP)
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Incoming Mail (cont’d.)
• “Forward root’s mail to” text box
– Root user account has full access to all Linux
machines on a network
– Logging in to mail server as a regular user with
limited permissions is recommended
– You can redirect system e-mails to your regular user
account by entering your username in “Forward
root’s mail to” text box
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Incoming Mail (cont’d.)
• Delivery Mode drop-down list
– Options
• Directly
• Through procmail
• To Cyrus IMAP Server
• Aliases button
– Used for redirecting e-mail to a different local user or
a list of local users
• Virtual domains button
– Used to create an alias for your domain name
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Incoming Mail (cont’d.)
• If you selected masquerading option in Outgoing
Mail window:
– You should configure a domain alias
• Activity 12-2: Configuring a Mail Server
– Configure a mail server to send e-mail from a user
on one computer to a user on another computer
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Incoming Mail (cont’d.)
Figure 12-5 The Forwarder Settings window
© Cengage Learning 2013
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Summary
• E-mail is based on standard protocols for sending,
storing, and receiving electronic messages
• Mail transfer agent delivers messages between
systems via Simple Mail Transfer Protocol
• Mail user agent is the mail application users run to
access mailboxes and send and receive e-mail
• SMTP is the standard protocol for sending e-mail
across a network
• Sendmail is the most widely used MTA on Linux
systems
•LinuxPostfix
is the preferred MTA in openSUSE
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Summary (cont’d.)
•
•
•
•
•
Qmail is a modular MTA
KMail and Evolution are two graphical MUAs
Mailx is a command-line MUA
Fetchmail is a text-based MUA
Can configure settings for an openSUSE mail
server with YaST Mail Server Configuration Wizard
• POP3 and IMAP are client/server protocols for
receiving e-mail
• Procmail: to filter and sort incoming e-mail
• Cyrus: a type of IMAP server
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