Postfix was written by Wietse Venema, who is widely known for his security tools and papers. It was made available as open source software in December 1998. IBM Research sponsored the initial release and has continued to support its ongoing development. (IBM calls the package Secure Mailer.) There were certain goals from the beginning that drove the design and development of Postfix:
Postfix shows its real value when operating under stressful conditions. Even within simple environments, software can encounter unexpected conditions. For example, many software systems behave unpredictably when they run out of memory or disk space. Postfix detects such conditions, and rather than make the problem worse, gives the system a chance to recover. Regardless of hazards thrown its way, Postfix takes every precaution to function in a stable and reliable way.
Postfix assumes it is running in a hostile environment. It employs multiple layers of defense to protect against attackers. The security concept of least privilege is employed throughout the Postfix system, so that each process, which can be run within an isolated compartment, runs with the lowest set of privileges it needs. Processes running with higher privileges never trust the unprivileged processes. Likewise, unneeded modules can be disabled, enhancing security and simplifying an installation.
Postfix was written with performance in mind and, in fact, takes steps to ensure that its speed doesn't overwhelm other systems. It uses techniques to limit both the number of new processes that have to be created and the number of filesystem accesses required in processing messages.
The Postfix system is actually made up of several different programs and subsystems. This approach allows for great flexibility. All of the pieces are easily tunable through straightforward configuration files.
Postfix is one of the easier email packages to set up and administer, as it uses straightforward configuration files and simple lookup tables for address translations and forwarding. The idea behind Postfix's configuration is the notion of least surprise, which means that, to the extent it's possible, Postfix behaves the way most people expect. When faced with design choices, Dr. Venema has opted for the decision that seems most reasonable to most humans.
Compatibility with Sendmail
With Sendmail compatibility, Postfix can easily replace Sendmail on a system without forcing any changes on users or breaking any of the applications that depend on it. Postfix supports Sendmail conventions like /etc/aliases and .forward files. The Sendmail executable program, sendmail, is replaced with a Postfix version that supports nearly all of the same command-line arguments but runs in conjunction with the Postfix system. While your Sendmail-dependent programs continue to work, Postfix has been evolving independently of Sendmail, and doesn't necessarily implement all email features in the same way.
General Configuration and Administration
Email and DNS
Local Delivery and POP/IMAP
Hosting Multiple Domains
Blocking Unsolicited Bulk Email
Transport Layer Security
Appendix A. Configuration Parameters
Appendix B. Postfix Commands
Appendix C. Compiling and Installing Postfix
Appendix D. Frequently Asked Questions