SquirrelMail is a PHP-based Web email client. It includes built-in pure PHP support for IMAP and SMTP, and renders all pages in pure HTML 4.0 for maximum compatibility across browsers. It has strong MIME support and a flexible plugin system.
AfterLogic WebMail Lite PHP is a free AJAX webmail application. Users can receive, view, manage, compose, and send email through web interface (IMAP and SMTP supported). Source code included.
Free and easy-to-use webmail script with AJAX interface. Supports POP3/IMAP accounts, SMTP, SSL. Users can check and send mail, use address book. Skins, non-English languages, UTF-8. MS SQL/MySQL/MS Access backends supported.
YAM - The E-mail client for Amiga computers
YAM (short for Yet Another Mailer) is a commonly used MIME-compliant Internet mailer written for AmigaOS computers. It supports POP3, SMTP, TLSv1/SSLv3, multiple users, PGP, hierarchical folders, filters, a configurable GUI, an ARexx interface and more...
Zarafa is a complete replacement for MS Exchange under Linux. Zarafa has a full featured AJAX based webaccess which is completely in the Outlook "Look & Feel".