Roundcube is a PHP and AJAX based Open Source webmail.

If you are looking for a greatest user experience with a free and open webmail, than Roundcube is the answer.

Like any other webmail, this solution can be used to connect user to any kind of mail message store (Cyrus, Courier, Zimbra, Horde, ...). And because it is using an AJAX based interface, the experience you got with it is almost similar to what you get with any other "fat client" like Thunderbird, Evolution or Microsoft Outlook.

One can argue that Roundcube has not yet reached the full mature level of a SquirrelMail solution, but the evolvment of its community, the rapid evolution of it and the great openess of its API for plugin make it a solution of choice for any organization looking for a webmail solution, coming into reenforcement of any existing fat client product.

For instance, while outside the network of the company, some Organization may prefer to give access to the corporate mail system with a webmail solution than the commonly used fat client inside of this organization.

RoundCube proposes a very complete API to develop plugin to extend functionality of it. Because it is AJAX based, the business logic inside the application (PHP) can be easily separated from the user interface logic (JavaScript) and gives opportunity to extend the interface and the functionalities to the infinite. RoundCube offers a set of so-called "hook" upon which plugin can be tied, making it easy to alter the standard behaviour of the product to suit your specific needs using plugins and thus whitout the need to modify the core programming of the product.