This plugin implements logging functionality for your webmail interface. 
You can choose to log to a database, a file, your system log, or any 
combination thereof. You can also choose which kinds of events to log, 
including login events, logout events, login error events, all outgoing 
messages, possible outgoing spam messages, and other error events.

Also included is monitoring functionality that will send alert emails to 
the administrator when certain events trigger.

If you use the timeout_user plugin, logout events caused by user timeouts 
will be captured.

Log message format is also completely custom-defined to meet your needs 
in the configuration file.

WWW: http://www.squirrelmail.org/plugin_view.php?id=52