Running Linux, 5th Edition - Matthias Kalle Dalheimer [439]
relayhost = [mail.example.org]
Additional Configurations
The configuration described here creates a simple Postfix installation to send and receive messages for users on your system. But Postfix is an extremely flexible MTA with many more configuration options, such as hosting multiple virtual domains, maintaining mailing lists, blocking spam, and scanning for viruses. The manpages, HTML files, and sample configuration files that come with Postfix contain a lot of information to guide you in the more advanced configurations.
Procmail
Being a celebrity on the Internet means that you get a lot of attention, just as celebrities do in the real world. The good news is that everyone can become celebrities: simply join a few public mailing lists, get yourself a home page, and you are all set. The bad news is that the attention is from spammers, who send you an enormous amount of suggestions about how you can become richer, extend certain body parts, and take most of their wealth if you want to help them get it out of Iraq.
The virtual bodyguards of your mail are a couple called Procmail and SpamAssassin. Procmail is a general-purpose mail filter, while SpamAssassin is a dedicated mail filter for fighting spam and the like (worms, viruses, etc.). This section discusses Procmail, and the next section is devoted to SpamAssassin.
Procmail Concepts
To understand Procmail, we need to start looking at how it is invoked. The usual sequence is that mail arrives at your account, and your MUA calls Procmail, giving it the mail as argument. The terms filter or rule, in many mail filtering programs, refer to both a set of conditions to check messages for and an action to perform on the messages that meet those conditions (such as putting them in a particular folder). Procmail refers to this set as a recipe, a term we will use throughout this section to describe each set of paired conditions and actions. Procmail goes through each of its recipes until one marks the mail as delivered. If no recipe blocks the mail, it is delivered in your inbox as if Procmail had never been in the picture.
Each recipe consists of two things: a set of conditions and a set of actions. The actions of a recipe are executed if all its conditions are met. In addition, a recipe may mark mail as delivered as described earlier.
The conditions may include the following:
The letter comes from president@whitehouse.gov.
The subject contain the text KimDaBa.
The body of the message contains the text The KDE Image Database.
All of the above.
The actions may include the following:
Reply to the sender that you are on holiday.
Forward the letter to another person.
Save the letter to a file.
Change some part of the letter (e.g., add a new header field, add some text to it etc.).
Before you dig too much into the details of this section, you should ask yourself if you really want to use Procmail at all. Many mail clients allow you to sort mail, and if we take KMail as an example, then it is much easier to use than Procmail. The following is a list of reasons why you may still want to use Procmail:
You are using a number of different mail clients, not always the same. For example, when you are on the road you use a web mail interface, but when you are home you use a normal mail client such as KMail or mutt.
You want to filter your mail the second it arrives, not at a later point when your mail client downloads it—an example of this may be out-of-the-office replies.
The amount of mail coming to your account is so big that you