System Administration Guide, Volume 3

Chapter 33 Introduction to Mail Services

Setting up and maintaining an electronic mail service are complex tasks, critical to the daily operation of your network. As network administrator, you might need to expand an existing mail service or perhaps set up mail service on a new network or subnet. To help you plan a mail service for your network, this chapter provides conceptual information about mail services and briefly outlines the tasks required for setting up typical mail configurations.

What's New With `sendmail`

Version 8.9.3 of sendmail has been included with the Solaris 8 release. Here is a list of the important or user-visible changes that are included in this new version:

A new configuration file option, called MaxHeadersLength, limits the length of the sum of all header lines in a given message. The default value is 32768 bytes. Incoming messages with headers that exceed this value are rejected.
A new file called /etc/default/sendmail can be used to store options to start sendmail with, rather than adding these options to the init script. The file makes it easier to upgrade systems, since the init scripts do not need to change.
The mail.local program has been extended to use the Local Mail Transfer Protocol. The protocol allows error codes to be returned for each recipient, so that the message is resent to just the recipients that did not receive the message rather than having to re-queue the message to all of the recipients. This protocol was added to sendmail in the Solaris 7 release.
A new command, named /usr/bin/praliases, can be used to turn the data in the alias database into plain text. If an argument is included on the command line, the command prints out a key:value pair, if the argument matches a key.
A new program called smrsh can be used to limit the number of commands that can be run using the "|program" syntax of sendmail. Only programs included in /var/adm/sm.bin can be run if this feature is enabled. Adding FEATURE(`smrsh') in the main configuration file enables this feature (see /usr/lib/mail/README for details.)
New options have been added to the vacation program: -f can be used to select an alternate database instead of ~/.vacation.ext; -m can be used to select an alternate message file instead of ~/.vacation.msg; and -s can be used to specify the reply address instead of the UNIX From line in the incoming message.
A change to the mailx program allows for the From: header to be used as the basis of the sender instead of the envelope sender. This change makes mailx work like mailtool and dtmail.

Additional information on the Solaris version of sendmail can be found at http://www.sendmail.org/sun-specific/migration+sun.html.

Other `sendmail` Information Sources

Here is a list of additional information sources about sendmail.

http://www.sendmail.org - Home page for sendmail
http://www.sendmail.org/faq - FAQ for sendmail
http://www.sendmail.org/m4/readme.html - README for new sendmail configuration files
http://www.sendmail.org/sun-specific/migration+sun.html - Differences between sendmail delivered with the 2.6 release and release 7.

Introduction to Mail Services Terminology

Many software and hardware components are required to establish a mail service. The following sections give a quick introduction to these components and some of the terminology used to describe them.

The first section defines the terminology used when discussing the software parts of the mail delivery system. The next section focuses on the functions of the hardware systems in a mail configuration.

Mail Services Software Components Overview

The following table introduces some of the software components of a mail system. See "Mail Services Software Terminology" for a complete description of all of the software components.

Component	Description
.forward files	Files that can be set up in a user's home directory to redirect mail or send mail to a program automatically
mailbox	A file on a mail server that is the final destination for email messages
mail addresses	Contains the name of the recipient and the system to which a mail message is delivered
mail aliases	An alternate name used in a mail address
mail queue	A collection of mail messages that needs to be processed by the mail server
postmaster	A special mail alias used to report problems and ask questions about the mail service
sendmail configuration file	A file that contains all the information necessary for mail routing

Hardware Components of a Mail Configuration

A mail configuration requires three elements, which can be combined on the same system or provided by separate systems:

A mail host - A system configured to handle email addresses that are difficult to resolve
At least one mail server - A system configured to hold one or more mailboxes
Mail clients - Systems that access mail from a mail server

When you want users to communicate with networks outside your domain, you must also add a fourth element, a mail gateway.

Figure 33-1 shows a typical electronic mail configuration, using the three basic mail elements plus a mail gateway.

Figure 33-1 Typical Electronic Mail Configuration

Each element is described in detail in "Hardware Components of a Mail Configuration".