The Mail Transfer Agent (MTA) is perhaps the most important part of a mail system. It is responsible for receiving messages from the Internet or from your own users and making sure that the messages safely arrive at their destinations—the other mail servers or mailboxes of your users.
Postfix has been chosen as the mail transfer agent to be covered in this book. Postfix has a large feature set, it has an excellent security track record, it is fast, easy to configure, and under active development.
This book assumes that you run Postfix 2.0 or later. Any feature or behavior of Postfix that is specific to later releases will be noted.