Which free webmail application is written in PHP and released under the GNU GPL?

Which free webmail application is written in PHP and released under the GNU GPL? SquirrelMail is the application described.

SquirrelMail is an open-source webmail interface written primarily in PHP. Rather than acting as a complete mail server, it provides a browser-based front end that connects to an IMAP server for stored messages and normally uses SMTP to send mail. Its core software and associated IMAP proxy component were released under the GNU General Public License.

Nathan and Luke Ehresman began the project in 1999. Its deliberately straightforward HTML interface made it useful on older browsers and helped it become a common choice in Linux distributions, hosting environments, and institutional mail systems. A plugin architecture extended the basic interface with features such as address books, filters, and other integrations.

Readers sometimes confuse SquirrelMail with Roundcube, another PHP webmail application. Roundcube arrived later, uses a more JavaScript-heavy interface, and eventually replaced SquirrelMail in some hosting-control-panel deployments. SquirrelMail’s last numbered stable release was version 1.4.22, published in July 2011, although later maintenance snapshots continued.