Where Can I Get Postfix?
Postfix is already available in many popular UNIX and Linux distributions, but it may not be enabled by default. Postfix comes as an optional install with Debian GNU/Linux; and is available in the OpenBSD, NetBSD, and FreeBSD ports collection. There are several third-party compiled RPMs for RedHat, SuSE, Mandrake, and other RPM-based GNU/Linux distributions, as well as packages for Apple's new MacOS X. And as always, you can install it from source on just about any flavor of UNIX with a C compiler.
The Postfix homepage, http://www.postfix.org/, contains tons of reference documentation, How-To documents, architectural documents, and the source code, along with links to mailing lists and mailing list archives.