Under the default Sendmail MTA installation, the default MDA is mail.local. mail.local is very limited in its capabilities. For example, many mail.local implementations in use today cannot perform a disk usage check (quota) prior to delivery or filter messages based on From:, To:, headers etc. This lack of functionality in mail.local led to Procmail's development and widespread use.

Fundamental Anti-Spam Techniques

In this section, we cover some of the common methods used to defeat spam. These techniques show up many times throughout this book and are the basis of the ongoing fight against spam

Inbound Versus Outbound Spam

Throughout this book, we use inbound and outbound to describe the direction of messages that your users receive and send, respectively. Inbound refers to messages (and spam) that end up in mailboxes on machines that you, as an administrator, manage. For example, these messages end up in IMAP or POP3 servers and are downloaded to an email client such as Mozilla or Microsoft Outlook. Outbound refers to messages that your users send from email clients, such as Microsoft Outlook Express or Lotus Notes, to remote users hosted on other mail systems. These messages (which hopefully are not spam!) pass through your mail systems on their way to their ultimate destination in some system not (necessarily) administrated by you.

This book focuses on inbound spam, or messages received by your users. Although Chapter 5, "SMTP AUTH and STARTTLS," is dedicated to stopping outbound spam, preventing your users from sending spam is usually a much easier problem to solve than managing the spam your users receive (inbound spam). The much more difficult problem to solve is inbound spam. Of course, you do not want to run email systems that are considered "open relays," which allow anyone on the Internet to send spam through them, so you must take steps to secure all mail servers appropriately against open relay access.