Roundcube

Roundcube is a free, open-source webmail application that provides a polished, desktop-like email experience through a web browser. It is one of the most widely deployed webmail clients in the hosting industry and serves as the default or alternative webmail option in cPanel, Plesk, and DirectAdmin.

The interface is clean and responsive, supporting drag-and-drop message management, threaded conversations, folder management, contact management with groups, identity management for multiple email addresses, and a rich text editor with inline image support. The user experience is significantly more modern than older webmail clients like SquirrelMail or Horde.

Roundcube connects to any IMAP server and supports SMTP for sending. It is compatible with Dovecot, Courier, Cyrus, and other standard IMAP servers used in hosting environments. The application is written in PHP and uses MySQL, PostgreSQL, or SQLite for storing user preferences, contacts, and session data.

The plugin architecture extends Roundcube's functionality significantly. Popular plugins add features like CardDAV contact sync, CalDAV calendar integration, two-factor authentication, encryption (PGP/GPG), vacation auto-responders, and custom skins. Many hosting providers install additional plugins to enhance the default experience for their customers.

For hosting providers, Roundcube is essentially a must-have. Customers expect to access their email through a web browser, and Roundcube provides a professional webmail experience at zero licensing cost. It integrates seamlessly with the email infrastructure already running on hosting servers.

cPanel includes Roundcube as one of its default webmail options alongside Horde. Plesk includes it as the primary webmail client. The application is actively maintained with regular security updates and feature improvements.