Configuring a Mail Server

Chapter 5: Configuring a Mail Server

Electronic mail is still the most important user service on the network. The Web carries a greater

volume of traffic, but e−mail is the service used for most person−to−person communication. And

person−to−person communication is the real foundation of business. No network is complete

without e−mail, and no network server operating system is worth its salt if it doesn't include full

TCP/IP mail support.

Simple Mail Transport Protocol (SMTP) is the TCP/IP mail transport protocol. Linux provides full

SMTP support through the sendmail program, although sendmail does more than just send and

receive SMTP mail. sendmail provides mail aliases and acts as a "mail router," routing mail from all

of the different user mail programs to the various mail delivery programs while ensuring that the mail

is properly formatted for delivery.

This chapter looks at your role in configuring each of these functions. Configuring sendmail can be a

large and complex task, but it doesn't have to be. Compared to some network server systems that

require a second installation just to install the SMTP server software, Linux distributions do a lot of

the configuration for you, and for most sites, the default configuration works fine. This chapter will

give you the information you need to make intelligent decisions about when and how to change the

default configuration.

sendmail configurations are built using the m4 macro processing language. The output of the m4

process is the sendmail.cf file, which is the configuration file read by sendmail. To fully understand

and manage sendmail, you need to understand its functions, the sendmail.cf file from which it reads

its configuration, and the m4 macros used to build that file. This chapter covers all three topics.

Using Mail Aliases

Mail aliases are defined in the aliases file. The location of the aliases file is set in the sendmail

configuration file. (You'll see this configuration file later in the chapter.) On Linux systems, the file is

usually located in the /etc directory (/etc/aliases), and it is occasionally located in the /etc/ mail

directory. The basic format of entries in the file is

alias

: recipient

The alias is the username in the e−mail address, and recipient is the name to which the mail should

be delivered. The recipient field can contain a username, another alias, or a final delivery address.

Additionally, there can be multiple recipients for a single alias.

sendmail aliases perform important functions that are an essential part of creating a mail server.

Mail aliases do the following:

Specify nicknames for individual users

addressed to special names, such as postmaster or root, to the real users that do

those jobs. When used in conjunction with the domain MX records covered in

Chapter 4, "Linux Name Services," aliases can be used to create a standard e−mail

address structure for a domain.Nicknames can be used to direct mail