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.
