The Postfix distribution ships with a lot of documentation. Depending on your installation package, you may or may not have all of the documents. You should have at least the manpages and sample configuration files. The sample files are located in the directory specified by the sample_directory parameter, which is usually the same directory in which your main.cf file resides. All of the Postfix parameters are documented in one or more of the sample files.
When Postfix was installed, the manpages should have been installed in a sensible place on your system. If they are in a directory where your system expects to find them, you only have to type, for example:
$ man postfix
to have the manpage displayed on your screen. If your system replies with an error message such as:
$ man postfix No manual entry found for postfix.
then either the pages are not installed or they are not in a location your system expects to find them. Read the documentation for your system to find out about setting your MANPATH variable or moving the manpages to a more standard location for your platform.
There are many manpages for various Postfix commands, daemons, and lookup tables. All of the documentation is also available as HTML files. If the HTML files are not installed on your system, you can find them on the Postfix web site at http://www.postfix.org/. The online documentation always refers to the current release of Postfix.
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