SquirrelMail is a web based email (webmail) client. It includes built-in pure PHP support for the IMAP and SMTP protocols, and all pages render in pure HTML 4.0 (with no JavaScript required) for maximum compatibility across browsers. SquirrelMail is Open Source.
SquirrelMail webmail uses a plugin architecture to accommodate additional features around the core application, and over 200 plugins are available on the SquirrelMail website. SquirrelMail has all the functionality of a desktop email client, including strong MIME support, address books, and folder manipulation, all in a web based email platform.