Andrew Hudson - Fedora™ Unleashed, 2008 edition

For example, you could

► Use Evolution to read and compose mail.

► Use Sendmail to send your mail.

► Use xbiff to notify you when you have new mail.

► Use Fetchmail to retrieve your mail from a remote mail server.

► Use Procmail to automatically sort your incoming mail based on sender, subject, or many other variables.

► Use Spamassassin to eliminate the unwanted messages before you read them.

Because Sendmail is the Fedora default client (and the mostly widely used client), the following sections provide a brief explanation and examples for configuring and operating your email system. As mentioned earlier, however, Sendmail is an extremely complex program with a very convoluted configuration. As such, this chapter covers only some of the basics. For more information on Sendmail, as well as other MTAs, see the "Reference" section at the end of this chapter.

Sendmail configuration is handled by files in the /etc/maildirectory, with much of the configuration being handled by the file sendmail.cf. The actual syntax of the configuration file, sendmail.cf,is cryptic (see the following example). In an attempt to make it easier to configure Sendmail, the sendmail.mcfile was created. The following example belies that goal, however. The sendmail.mcfile must be processed with the m4 macro processor to create the sendmail.cffile; the needs of that processor account for the unusual syntax of the file. You will learn how to use it later, and we see a Perl script that automates and simplifies the entire process. First, let's examine some basic configuration you might want to do with Sendmail.

sendmail.cfhas some strange syntax because of the requirements of the m4macro processor. You do not need to understand the details of m4here, but note the quoting system. The starting quote is a backtick ( `), and the ending quote is simply a single quote ( '). Also, the dnlsequence means to "delete to new line" and causes anything from the sequence up to and including the newline character to be deleted in the output.

Here's a look at an excerpt from the sendmail.cffile:

CP.

# "Smart" relay host (may be null)

DS

# operators that cannot be in local usernames (i.e., network indicators)

CO @ % !

# a class with just dot (for identifying canonical names)

C..

# a class with just a left bracket (for identifying domain literals) C[[

# access_db acceptance class

C{Accept}OK RELAY

C{ResOk}OKR

# Hosts for which relaying is permitted ($=R)

FR-o /etc/mail/relay-domains

And here's a quote from the sendmail.mcfile for comparison:

dnl define(`SMART_HOST',`smtp.your.provider')

define(`confDEF_USER_ID',``8:12'')dnl

undefine(`UUCP_RELAY')dnl

undefine(`BITNET_RELAY')dnl

dnl define(`confAUTO_REBUILD')dnl

define(`confTO_CONNECT', 4m')dnl

define(`confTRY_NULL_MX_LIST',true)dnl

You can see why the file is described as cryptic.

Complicated email server setup is beyond the scope of this book; for more information on this topic, we suggest Sendmail, 3rd Edition by Costales and Allman, a 1,200-page comprehensive tome on Sendmail configuration. However, the following five sections address some commonly used advanced options.

Sometimes you might want to have Sendmail masquerade as a host other than the actual hostname of your system. Such a situation could occur if you have a dialup connection to the Internet and your ISP handles all your mail for you. In this case, you want Sendmail to masquerade as the domain name of your ISP. For example:

MASQUERADE_AS(`samplenet.org')dnl

If you do not have a full-time connection to the Internet, you probably want to have Sendmail send your messages to your ISP's mail server and let it handle delivery for you. Without a full-time Internet connection, you could find it difficult to deliver messages to some locations (such as some underdeveloped areas of the world where email services are unreliable and sporadic). In those situations, you can configure Sendmail to function as a smart host by passing email on to another sender rather than attempting to deliver the email directly. You can use a line such as the following in the sendmail.mcfile to enable a smart host:

define(`SMART_HOST', `smtp.samplenet.org')

This line causes Sendmail to pass any mail it receives to the server smtp.samplenet.orgrather than attempt to deliver it directly. Smart hosting will not work for you if your ISP, like many others, blocks any mail relaying. Some ISPs block relaying because it is frequently used to disseminate spam.

As mentioned earlier, Sendmail typically attempts to deliver messages as soon as it receives them, and again at regular intervals after that. If you have only periodic connections to the Internet, as with a dialup connection, you likely would prefer that Sendmail hold all messages in the queue and attempt to deliver them at specific time intervals or at your prompt. You can configure Sendmail to do so by adding the following line to sendmail.mc:

define(`confDELIVERY_MODE', `d')dnl

This line causes Sendmail to attempt mail delivery only at regularly scheduled queue processing intervals (by default, somewhere between 20 and 30 minutes).

However, this delay time might not be sufficient if you are offline for longer periods of time. In those situations, you can invoke Sendmail with no queue processing time. For example, by default, Sendmail might start with the following command:

# sendmail -bd -q30m

This tells Sendmail that it should process the mail queue (and attempt message delivery) every 30 minutes. You can change 30 to any other number to change the delivery interval. If you want Sendmail to wait for a specific prompt before processing the queue, you can invoke Sendmail with no queue time, like this:

# sendmail -bd -q

This command tells Sendmail to process the queue once when it is started, and again only when you manually direct it to do so. To manually tell Sendmail to process the queue, you can use a command like the following:

# sendmail -q

If you use networking over a modem, there is a configuration file for pppdcalled ppp.linkup, which is located in /etc/ppp. Any commands in this file are automatically run each time the PPP daemon is started. You can add the line sendmail -qto this file to have your mail queue automatically processed each time you dial up your Internet connection.

Books are available to explore the depths of Sendmail configuration, but the Sendmail Installation and Operation Guide (check on Google for this) is the canonical reference. Configuration guidance can also be found through a Google search; many people use Sendmail in many different configurations. Fortunately, Fedora has provided a default Sendmail configuration that works out of the box for a home user as long as your networking is correctly configured and you do not require an ISP-like Sendmail configuration.