20.1. Introduction

The Linux world has many excellent programs for handling email: Sendmail, Exim, qmail, and Postfix are the top four mail transfer agents (MTAs). This chapter covers Postfix. Like most of the post-Sendmail generation of MTAs, Postfix is designed from the ground up to be secureable and robust. It scales nicely from the single user who wants more control over her personal mail all the way up to the largest service provider.

Here's a bit of terminology:

MTA

Mail transfer agent. This moves email between servers. Sendmail, Exim, qmail, and Postfix are MTAs. An MTA must support SMTP.

SMTP

Simple Mail Transfer Protocol. This moves messages between mail servers.

MUA

Mail user agent, also called "mail client." Mutt, Pine, Kmail, Evolution, and Balsa are MUAs. This is the user's program for composing, sending, and receiving email. MUAs can fetch mail from a local folder, or from a remote server via POP and IMAP.

MDA

Mail delivery agent. This is an intermediary between an MTA and a MUA. Procmail and Fetchmail are two popular MDAs. An MDA is not required; it is used for extra functionality, such as filtering, sorting, and autoresponding.

POP

Post Office Protocol. Moves messages from the server to the user's inbox. A POP server is simple to operate and does not need a lot of horsepower.

IMAP

Interactive Message Access Protocol. The message store remains on the server. An IMAP server needs a lot of RAM and lots of storage space.

TLS

Transport Layer Security is an evolution of SSL (Secure Sockets Layer). It provides encrypted transport for SASL-authenticated logins.

SASL

Simple Authentication and Security Layer, for authenticating users. SASL does the authenticating, then TLS provides the encrypted transport of the authentication data.

These are the official ports for different mail protocols:

tcp/25

SMTP

tcp/110

POP3

tcp/995

POP3 over SSL

tcp/143

IMAP

tcp/993

IMAP over SSL

There are several ways to build a Linux mail server. Most admins take the modular approach and build it from a collection of specialized programs. That is what we will do in this chapter. Another option is to use the Courier package, because it is a complete package that contains an MTA, POP3, IMAP, and a mailing list manager. Or purchase a distribution that puts it all together for you, like SuSE OpenExchange.