This plugin will place "Spam" and/or "Not Spam" buttons on the mailbox message
list page as well as on a single message view page. The action associated with
the buttons (as well as the button text) can be configured to suit most any
spam reporting system. Reporting by email, reporting by executing a command on
the server and reporting by moving (or copying) the message to a designated
folder are all supported. Any number of custom buttons may also be added, where
the associated action is completely customizable (for instance, adding the
message sender to a whitelist or blacklist).

WWW: http://www.squirrelmail.org/plugin_view.php?id=242