phlyMail is a groupware, webmailer, and PIM. It offers unlimited nested groups, unlimited POP3/IMAP profiles, quotas, an integrated address book, a calendar with multiple reminders and event repetitions, a file manager with an integrated WebDAV server, the ability to send faxes, SMS support (which lets you send SMS, get reminded of events via SMS, and get email notification via SMS), a bookmark manager, themes, and multilingual support. It uses either sendmail or SMTP to send email, and can handle MIME and SSL connections. Support for IDN (Internationalized Domain Names) is included. Through its modular design, it's easy to add completely new features as modules, which can be plugged into the system.
Hastymail is a full featured IMAP/SMTP client written in PHP. The goal is a full featured yet lightweight Web mail client. After a complete review and rewrite, it provides many improvements over the original code including optional AJAX functionality, improved code organization, a plugin API, CSS and PHP template themes, and much more.
EMEXIS WEBMAIL is a Webmail application based on SquirrelMail. SquirrelMail client is a complete Webmail system, but due to the concern of making it compatible with many browsers and systems, its interface has become a little impaired. EMEXIS WEBMAIL was developed with the intention of solving this deficiency. It comes with all functionality of the original Squirelmail, but with an interface that is much more friendly for the user. It mostly uses the mechanisms for handling messages from Squirrelmail.