Roundcube

Roundcube Webmail is a multilingual IMAP client that runs in the browser and behaves like a desktop application. It offers the functions people expect from an email client, such as MIME support, an address book, folder management, message search and spell checking, without installing anything on their own computers.

It is written in PHP and JavaScript and requires MariaDB, MySQL, PostgreSQL or SQLite as its database. A plugin API makes it extensible, and the user interface can be customized with skins. The client uses jQuery 3.x and inherits its browser support, covering current versions of Chrome, Edge, Firefox and Safari. Releases follow semantic versioning, and the README ships separate install and upgrade guides.

Roundcube is licensed under GPL-3.0 and is installed on a web server that connects to an existing IMAP mail server. Note that the repository README warns that the Git snapshot is not a stable version and should not replace a production installation.