SquirrelMail is a standards-based webmail package written in PHP. It includes built-in pure PHP support for the IMAP and SMTP protocols, and is designed for maximum compatibility across browsers. SquirrelMail has few requirements and is easy to configure and install. It runs on top of any IMAP server.

SquirrelMail has all the functionality you would want from an email client, including strong MIME support, address books and folder manipulation.