Running Linux, 5th Edition

Filtering Spam

The constant flood of so-called spam (more precisely, unsolicited commercial email) has decreased the usefulness of email as a communication medium considerably. Luckily, there are tools that can help us with that as well. These are called spam filters, and what they do is to attempt to categorize each incoming message according to a large number of rules to determine whether it is spam. The filters then mark up the message with either certain additional header lines or a changed subject line. It is then your task (or your mail user agent's task) to sort the messages according to these criteria into separate folders (or, quite dangerously, into the trash can directly). At the end of the day, you decide how aggressively you want to handle spam. You need to make up your mind what is more important to you: to filter out as much spam as possible, or to ensure that no important message (such as a request from a potential customer) will ever get filtered out.

There are two different ways of using a spam filter: either directly on the mail server, or in your email client. Filtering directly on the mail server is advantageous if the mail server serves more than one mail client, because then the same set of filtering rules can be applied and maintained for all users connected to this mail server, and a message coming in to several users on this server only needs to pass the spam filter once, which saves processing time. On the other hand, filtering on the client side allows ...