Chapter 15. Email

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which grew out of an earlier email delivery system called Delivermail, quickly became the standard

as the email began to expand and become widely used.

15.3.2. Purpose and Limitations

It is important to be aware of what Sendmail is and what it can do for you as opposed to what it is

not. In these days of monolithic applications that fulfill multiple roles, you might initially think that

Sendmail is the only application you need to run an email server within your organization. Technically,

this is true, as Sendmail can spool mail to your users' directories and accepts new email via the

command line. But, most users actually require much more than simple email delivery. They usually

want to interact with their email using an MUA that uses POP or IMAP to download their messages to

their local machine. Or, they may prefer a Web interface to gain access to their mailbox. These other

applications can work in conjunction with Sendmail and SMTP, but they actually exist for different

reasons and can operate separately from one another.

It is beyond the scope of this section to go into all that Sendmail should or could be configured to

do. With literally hundreds of different options and rule sets, entire volumes are dedicated to helping

explain everything that can be done and how to fix things that go wrong. You should consult the many

excellent resources on Sendmail in order to shape it to fit your exact specifications.

However, you should understand what files are installed with Sendmail by default on your system and

know how to make basic configuration changes. You should also be aware of how to stop unwanted

email (spam) and how to extend Sendmail with the Lightweight Directory Access Protocol (LDAP).

15.3.3. The Default Sendmail Installation

While you can download the source code for Sendmail and build your own copy, many users prefer

to use the version of Sendmail installed by default with their Red Hat Linux system. You can also use

the Red Hat Linux CD ROMs to reinstall the

sendmail

RPM at a later point. Be aware that you must

alter the default configuration file for Sendmail to use it as a mail server for more the localhost. See

Section 15.3.4 for details.

After installation, the

sendmail

executable is placed in the

/usr/sbin/

directory.

Sendmail's lengthy and detailed configuration file,

sendmail.cf

, is installed in the

/etc/mail/

directory. You should avoid editing the

sendmail.cf

file directly. Instead, to make

configuration changes to Sendmail, edit the

/etc/mail/sendmail.mc

file, back up the original

/etc/mail/sendmail.cf

, and then use the included

m4

macro processor to create a new

/etc/mail/sendmail.cf

. More information on configuring Sendmail can be found in Section

15.3.4.

Various Sendmail configuration files are installed in the

/etc/mail/

directory including:

  access

  Specifies which systems can use Sendmail for relaying email.

  domaintable

  Allows you to provide domain name mapping.

  local host names

  The place where you include all aliases for your machine.

  mailertable

  Specifies instructions that override routing for particular domains.

  virtusertable

  Permits you to do a domain specific form of aliasing, allowing multiple virtual

domains to be hosted on one machine.

Several of the configuration files in

/etc/mail/

, such as

access

,

domaintable

,

mailertable

and

virtusertable

, must actually store their information in database files before Sendmail can use

any configuration changes. To include any changes you make to these configurations in their database

files, you must run the

makemap hash /etc/mail/ name

/etc/mail/ name

command,

where

name

is the name of the configuration file to convert.