courier-auth-postfix allows administrators to restrict relaying only to those users that successfully accessed their mailbox either with pop3 or imap (that successfully authenticated against courier-imap or courier-pop3, aka pop-before-smtp).
It was meant for those internet providers that do not want to leave their smtp server as ``open-relay'' but do not want to use smtp user authentication and do not want to set restrictions based on IP addresses.
courier-auth-postfix is provided as a patch for postfix, but, strictly speaking, it is not a real patch. It is a standalone module that is provided as a patch just to simplify the task of compiling it. Postfix nor courier would be modified by this patch and it would be possible to compile everything independently. However, no support is provided in this manual (not too hard anyway -- good luck). From courier point of view, you will end up with an additional authentication module that can be used exactly like any other authentication module (help will be provided in the following sections).
In order to optimize the usage of some kind of databases and in order to provide additional features, courier-auth-postfix supports ``optimizers''. An optimizer is just a small module compiled in courier-auth-postfix able to handle requests more efficiently. Actually, the only optimizer provided adds complete support for MySql (a delete query is much faster than scanning all records).
Once compiled, in order to use courier-authpostfix, you need to modify just a couple of lines in main.cf and in courier configuration files (mainly pop3d and imapd).
You can find the latest version of this document and the mentioned software at http://www.commedia.it/ccontavalli/. If you have troubles/suggestions/corrections feel free to mail me at <ccontavalli at commedia.it>.