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.

SquirrelMail is a standards-based webmail package written in PHP. It runs on top of any IMAP server.