Plugin:
In order to use the authorization features, you must be running the Squirrelmail Devel CVS version as of at least 10th of October 2006.
The authorization features in Squirrelmail IMAP backend will be available since release 1.5.2.
Note that the CVS snapshots of squirrelmail devel branch (1.5.x) might be unstable or have some functionality that occasionally does not work well. Therefore it is not advised to use this in a production environment.
You can grab the latest sources via SVN, as follows:
svn co https://email.uoa.gr/repos/squirrelmail/plugins/authz/trunk authz
Web interface to Subversion Code Repository
Just untar it under Squirrelmail's plugin directory, and use configure/conf.pl or ./configure or vi config/config.php to add the plugin as usual.
Afterwards, copy config_sample.php to config.php and change the default values in case they do not satisfy you. This file is self documented with comments.