Postfix was released under the IBM Public License. All Postfix source code is signed with Wietse's PGP key.
Stable releases do not change except for bugfixes and for portability fixes. New features are tested out in experimental releases (see below).
Stable releases are called "Postfix a.b.c", where a is the major release number, b is the minor release number, and c is the patchlevel.
Release candidates are made available so that people can adopt fixes before a new Postfix stable release comes out. The code may still change as new defects are discovered.
New features are tested in experimental releases. They become part of the next official release once the code has not changed for a significant amount of time. Although this code is still subject to change, it runs on all of Wietse's systems so it is production quality.
Experimental releases are called "Postfix a.b-yyyymmdd", where a.b is the next official Postfix release and yyyymmdd is the release date.
Patch to black-list domain names by their mail servers (such as Verisign's mail sink) or by their DNS servers (second revision: don't defer mail when DNS lookup fails; lookup grandparent NS records).